We had both always dreamed of going to South America; reading Che Guevara's The Motorcycle Diaries, the classic account of his 7,000-mile journey through the deserts and mountains of South America was exactly the kind of inspiration we needed. We were both 23, the same age as Che had been at the time, and had time on our hands after finishing university. The fact that we had never ridden motorcycles before didn't seem important, and we began planning and publicising in earnest.

Since Che was so moved by meeting the patients in Peruvian leprosy colonies en route, we decided our journey would mark the 75th anniversary of LEPRA, the British Leprosy Relief Association.

One year later, we found ourselves in Buenos Aires with motorbikes we were barely able to ride. We loaded up and headed for the town of Alta Gracia, where Che was brought up. Our arrival caused quite a stir, attracting local television and radio coverage. We interviewed Che's ageing yet sprightly primary school teacher, Elba Rossi, who gave us sweet cherry liquor and recounted tales of the future revolutionary. A surprisingly normal boy it seemed, with a penchant for stealing peaches from other people's gardens.

Back in the capital, we felt ready to follow the trail blazed by Che, leaving 47 years and five months to the day that both he and his friend Alberto set off in January 1952, heading down the flat Atlantic coast. At the town of Bahiá Blanca, we turned inland and rode into the heart of the vast Pampas. Still on good roads, we made rapid progress through the wastes of thorny scrub until the increasing cold and the short daylight hours of the southern hemisphere winter began to limit our riding time. By the time San Martin de los Andes and the epony mous foothills which surround it loomed large in front of our eyes, we had been frozen to the core on more than one occasion.

As the cold nights intensified, our five-litre water sack would freeze - a fate that also befell the contents of the Thermos flask and, to our annoyance, the washing up liquid bottle. The clarity of the stars in the clear, dry air would have been a consolation had we been warm enough to stay out for any length of time to appreciate them in all their beauty.

Our decision to travel light meant we had denied ourselves the luxury of a tent, instead packing two khaki German army ponchos that we clipped together and slung between the bikes to make a rudimentary shelter. This proved far from satisfactory, and we'd spend the long frozen nights buried at the bottom of our sleeping bags, still wearing all our clothes, boots and riding gear.

The Southern Argentine Andes in June are somewhat reminiscent of the English Lake District in winter - great barren, snow-clad mountains, expanses of rock interspersed with glacial lakes. It was only when both ourselves and the road became snow clad that we realised how bitter the conditions could be.

Fortunately, however, we soon left behind Argentina's bleak wastes and crossed the border into Chile, where the lush dank vegetation was a welcome relief. However this proved to be short-lived, it quickly becoming apparent that the reason for Southern Chile's verdancy was a ridiculous amount of rain. Our first stop, the town of Osorno, has an average of 200 days rain a year, typically concentrated at the time we were there.

We spent two days sheltering in a barn with a bronchial and homeless ex-trucker named Gonzalez. Heartily sharing our suppers of pasta and beer, he explained how most people in Chile hated General Pinochet and why Rolls-Royce represented everything that was good about Britain.

After 48 hours we realised the chances of a break in the downpour were remote and headed off to Puerto Montt, Chile's most southerly town with road access to the north. We were still 2,000 miles south of the border crossing into tropical Peru and it felt like it.

The murky clouds which obscured the classically beautiful pyramid peak of Volcan Osorno set the scene for the next two weeks. Progress was always slow, with frequent stops needed in order to warm our bones.

In such conditions, the disadvantages of our ponchos really became apparent. The moisture content of our sleeping area was little helped by the hoods each poncho sported in the middle, and come morning we rarely awoke dry and warm. Our appearance went steadily down hill: more than once a friendly stranger took pity and offered us the use of their washing facilities.

Then one of the bikes gave up the ghost and left us stranded in the grey town of Los Angeles. Our mechanical ineptitude ensured that not only did we not know what caused the problem but that even when we had located a worn out spark plug with the help of a friendly mechanic, we found that we had brought the wrong spares.

We must have looked pretty sorry for ourselves since the mechanic, Francisco, lit a charcoal brazier to dry our sodden clothes and, having offered us accommodation on the workshop floor, proceeded to bedeck the fire with the beginnings of surely the greatest barbecue in the history of mankind. Soon the heady gin-like Chilean national drink of Pisco made an appearance, and the rest of the evening is a blur.

Los Angeles marked a change of fortune, and on the road to the capital, Santiago, we received just one more severe drenching. Indeed, from Santiago onwards we experienced only five minutes of light drizzle in the next 3,500 miles.

After a short spell in the dull Europeanised capital it was a relief to head up the rugged winding Pacific coast Easy Rider-style in bright sunshine. Soon, the scenery metamorphosed from grassland to scrub, from scrub to cactus and from cactus to sand, and we found ourselves on the edge of the driest desert in the world.

The Atacama desert is not to be taken lightly, and certainly not on motorbikes with a meagre 100-mile range, so we acquired spare jerry cans and extra water bottles in the picturesque colonial centre of La Serena. The town was founded in 1544 by the Spanish conquistador Juan Buhon following his hellish crossing of the Atacama, in which more than half the conquistadors and their native slaves died. It was with some trepidation that we headed for the desert.

In the bleak wastes of the Atacama, the shimmering road ahead was our only link with civilisation. Throughout the day we passed ghost towns, roofless houses alone in the desolate haze. In the breathless midday heat we stopped at one of these old nitrate mining settlements, Officina Edwards. Searching for shade in one of its crumbling buildings, the interior walls still painted a faded yellow and maroon, we found an immaculate 1920s child's leather shoe, delicately stitched and nailed, perfectly preserved in the dry air. Through a paneless, shutterless window, out across the hot sand, a solitary cross marked a forgotten grave. We sat listlessly in the relentless heat, sipping our precious water and tried to imagine the harsh life of the mining families that the dusty remains once housed.

It was in the desert wastes that Che, (like any a good prophet should) had one of the revelations that shaped his life. One bitterly cold night spent sharing a "miserable blanket" with a persecuted communist miner and his wife was an experience that made Che feel closer, really for the first time, to the marginalised people he would later come to fight for: those he then termed this "strange, for me at least, human species".

Back on the road, we pushed on across a barren expanse of flat, grey desert bounded by the spectacular volcanoes that mark Chile's border with Bolivia; at the oasis village of San Pedro de Atacama we left the road behind. We felt truly free bumping along the rutted gravely tracks. The dust billowed out behind us and there was nothing in sight except parched bare-backed mountains that rose up from the plain.

That night, we had no need to erect our ponchos as we found our way into the Valley of the Moon, a fantastic selection of wind sculpted rocks and dunes.

Nevertheless, real adventure beckoned, and we set course for the El Tatio geyser field on the side of one of the distant volcanoes that mark the border with Bolivia. The road was terrible, and we bounced and shook our way upwards throughout the day, the air becoming thinner and colder by the minute. It proved to be the most spec tacular period of riding we had experienced.

When we reached the geysers in the late afternoon we were exhausted but elated. We had made it to 15,000 feet on our struggling bikes, vicuñas (wild relations of the llama) running out in front of us from frozen lakes, the dust from their hooves hanging ghostlike in the cold dry air long after our passing.

The geysers turned out to be gently bubbling steaming pools in which we boiled eggs for breakfast, frozen and feeling like death because of altitude sickness. Much as we would have liked to stay to appreciate the stunning scenery, we feared for our health and hastily descended.

Back on the main roads we made speedy progress towards mountainous Peru. The sun shone brightly everyday, and although the desert nights were bitterly cold, we could sleep without shelter by the side of our bikes.

Peru proved completely different to anything we had expected. Within 60 miles of the border we were ascending steadily on ancient roads, little more than donkey tracks, through a land that time had forgotten, epitomised by the ruins of the Inca citadel of Machu Picchu.

Three months after setting out, we made it into the polluted heart of Lima, our faces looking like chimney sweeps from all the soot we had accumulated in our helmets along the way.

In Lima, Che stayed with a famous leprologist, Dr Hugo Pesce and was greatly moved by the patients he met there. He later headed north into the jungle, where he spent his 24th birthday with the leprosy patients of the San Pablo colony on the Amazon; Che was on the cusp of his true vocation.

•BBC Radio 4 will broadcast the story of Ben and Karl's journey on Tuesday at 11am as part of its six-part series of travellers' stories, Tales from the Back of Beyond. The Motorcycle Diaries, by Che Guevara, is published by Fourth Estate, 1996, price £5.99 (ISBN 185702 399 4).