It was third time lucky for Philip Campbell. The 60-year- old from Perthshire had first approached Voluntary Service Overseas (VSO) in 1969 as a new graduate. "I told them my degree was philosophy and they said, go away and do something useful," he says, with a laugh. He went back in 1993, after 23 years as a journalist. Once again, there were no suitable vacancies.

But when he went back in 2006 after seven years as a religious education and special needs teacher in London, VSO had just the job for him. So in October 2006, he found himself saying goodbye to his wife Sarah and heading off to Thailand for a 16-month posting at Mae Sot on the Thai-Burmese border, working to improve the education of Burmese refugee children against the background of a deteriorating political situation in Burma. He kept in touch with home through an online blog. By the end of his posting, he had made alliances with Burmese teachers and Thai government officials, lobbied the Thai Queen, and helped achieve a crucial objective: getting ID cards for migrant teachers, so they no longer had to live in fear of deportation.

It's all in a year's work for VSO volunteers. There have been 30,000 of them since VSO, celebrating its half century this year, was set up by former journalist and refugee worker Alec Dickson and his wife Mora, in 1958. Currently, VSO has 1500 professionals working in 34 countries. They include doctors, nurses and teachers but also social workers, small business advisers and farmers. Many past Scottish volunteers will come together in Edinburgh on March 1 for an anniversary celebration. It will be a chance for volunteers from VSO's earliest days to compare notes with their 21st-century counterparts.

They will have much to talk about. The aim of VSO (motto: Sharing Skills, Changing Lives) has always been for volunteers to pass on their skills to local people. But the last school leaver was posted on a standard placement 35 years ago: now the average age of volunteers is 41. Placements are no longer set at one or two years, but might just be a matter of weeks. And in the early days, there was no such thing as e-mail and internet blogs.

"You were truly cut off," says Magdalene Sacranie, who went to Malawi in 1968 as a 23-year-old physiotherapist. Magdalene, who lives in Perthshire, signed up for VSO once she had two years' post-grad experience. "I was quite nervous, but completely convinced it was the only thing I wanted to do," she says.

It was only four years since Malawi had gained independence and many remaining British personnel still enjoyed the "sundowner" expatriate lifestyle, though Magdalene found many highly committed and remembers them with affection. Among them was Dr David Molesworth, a former prisoner of war who had dedicated his life to leprosy care. "He headed up the Lepra project," says Magdalene. "He strode about in the familiar khakis, with a full moustache and beard. He looked like James Robertson Justice." Magdalene still remembers him referring to "Darkest A" - the A standing for Africa - the old colonial formulation.

Her work was with the Lepra project, based in the capital Blantyre. Its aim was to treat all the leprosy sufferers within a 10,000 square mile project area as out-patients, managing their illness with drugs. "It was so ahead of its time," she says.

Her job involved going out to the various rural outposts where sufferers were left to live out their days. "I couldn't believe it: what were all these brick privvies with corrugated iron roofs doing on the hillside? Then I realised: that's where they lived. It was unbelievably cruel. People would go there for life and no one would ever see them again."

Magdalene's role was to come up with ways to treat the patients and also help them develop vocational skills so they could be reintegrated into village life. Her training was some help when it came to making prostheses but setting up a workshop for the patients took raw determination. She begged what she could off local businessmen, including four treadle sewing machines, and bought simple screen-printing equipment so they could make stationery to sell at the Leprosy Craft Shop. On top of that, she trained workers in rehabilitation techniques.

Magdalene's work in Malawi made a measurable difference to the lives of leprosy patients there. But it was also significant personally. Going into the Central Bookshop in Blantyre one day, she got talking to the man behind the counter, Aziz. They were married two months after arriving back in the UK - 36 years ago. She and Aziz still have a lot of family in Malawi and support a charity co-founded by Aziz's brother and his wife, the Children's Fund of Malawi, whose work includes assisting "granny families" (where the parents have died from AIDS) and upgrading orphanages. To raise funds, Magdalene has written a book called African Dreamtime, recording 50 African folk wisdom stories, and is currently seeking a publisher.

She believes that the fact VSO asks countries to identify what they want from their VSO volunteers has been crucial to its success. "It's a partnership right from the beginning." Philip agrees. "VSO had researched my placement extremely well," he says. "It's about making sure there's a practical, achieveable end."

Philip was brought out because someone independent was needed to help build a relationship between the Thai authorities and the Burmese camps. The Thai government was unwilling to grant nationality to so many refugees (an estimated 300,000) but was nevertheless committed to helping educate refugee children, being taught in "illegal" schools. ID cards for the migrant teachers, to prevent them from being deported, would help. Philip, working with committed Thai officials, made this his goal. He tracked down the Thai Queen when she visited the area, handing her a letter, but the breakthrough finally came when the Education Minister took up the cause. Thirteen months after he'd arrived, Philip found himself in a meeting with Thai government officials drawing up a new policy on Burmese schools. He ensured the issue of ID cards was included. They were handed out to staff this month - just after Philip had left Thailand. The registration cards, as they are called, do not have legal status, but soldiers are encouraged not to arrest their holders.

Philip, like many VSO volunteers, had some trepidation before leaving for Thailand about whether his skills would equip him for the job. He found that they did. "My job was so much a matter of advocacy. I was on Newsnight for many years with John Tusa and Peter Snow, so I'd spent a lot of my time cajoling and organising."

Could a local have done the job just as easily? Not in this case because of the sensitivities involved - that was precisely why a VSO volunteer was requested for the role. "A Thai couldn't have done it and nor could a Burmese."

VSO has in the past been sneered at, especially in its early post-colonial days, as being an adventure for volunteers. But for Andrew Cubie, the chairman of the UK Committee of VSO, while volunteers certainly benefit from it, that is very much secondary to the benefit that they bring to the countries they visit.

"The people who volunteer are skilled in what they do. That is of enormous importance," he says. "This is not to be regarded as anything except a really significant contribution to these countries."

Among VSO's achievements in the last half century, he highlights the fact that they are welcomed into the poorest countries in the world, including former colonies, with open arms. For instance, the Rwandan government invited the organisation to form an educational structure for the country after the genocide, during which two thirds of the counrty's teachers were murdered. Increasingly, volunteers are involved in ongoing programmes rather than one-off projects. VSO is also assisting governments in several countries with their own national volunteer programmes.

Cubie is delighted at the former and current Scottish government's commitment to the NHS Scotland partnership, whereby health workers can work in Africa and keep their employment and health benefits intact. The Scottish government has backed this commitment with funding.

Last year, VSO's UK director, Judith Brodie, vented her concerns about the number of "spurious" gap-year volunteer schemes on offer, saying that while there were many good providers, some ultimately benefited no one but the travel companies organising them.

"We really are not in competition with agencies like that," says Cubie. "Our UK director was quite critical of some of them because they are billed as being an experience for the volunteer, but were not focused on the countries." VSO is guided by the countries as to what they want, though the benefit is mutual.

As Magdalene says of Malawi: "There's a place in my heart to see things come right in this beautiful country with its beautiful people, something that has been with me all these years." Your VSO: Celebrating 50 years of VSO is on Saturday, March 1, from 2pm at the Hub, the Royal Mile, Edinburgh. For tickets, visit www.vso.org.uk/50th or call 020 8780 7500.