I could have been more cautious but you must call it as you see it

Dunion: "I'm information commissioner."

Taxi driver: "What's that then?"

Dunion: "I'm the man who ordered the release of MSPs' expenses."

And suddenly, the driver gets it; suddenly, he knows who is in the back of his cab: a man who will stand up to those pocket-filling politicians, a radical in a sensible suit, a respectable troublemaker.

And that phrase Dunion uses to describe these encounters – "taxi moment" – has an extra relevance for Scotland's first information commissioner because the MSPs' expenses he ordered to be released included those of the former Scottish Tory leader David McLetchie– and they were, of course, for taxis.

Dunion, 56, says that of all the decisions he has made as information commissioner, it's that one on the taxi expenses in 2005 that seems to have most connected him to the public. Before the decision, which led to Mr McLetchie's resignation, very few people had heard of FOI, but after it, most had, and, what's more, they realised what it could mean for them. The public saw the law in real-time, says Dunion, and in prime-time. "From that point of view," he says, "it was a seminal moment."

Not that Dunion sees his career as a series of seminal moments. In fact, he sees it as an ongoing project even though he's leaving the job this week after nine years. Sitting in his bright, light office in St Andrews ahead of his last day on Thursday, he tells me he's off to India shortly, and then Brazil offering advice and guidance on FOI – and a warning too.

The warning is that it's all very well setting up a law giving citizens the right to access information, but you must also guard against backsliding. And that can happen anywhere, says Dunion. In fact, it's happening right now in Scotland.

The problem in this country, he says, is that, increasingly, public authorities are handing over responsibilities to trusts or private bodies and they are spending public money but are not subject to FOI. The solution, Dunion says, is that the public's right to information should follow the public money.

It's obvious Dunion is still passionate about this issue, and I ask him why that is. Here he is at the end of two terms as commissioner and he's still pushing. Where does this trouble-making come from? Isn't radicalism something for men in their twenties rather than their late fifties?

"It's just in the DNA," he says. "You do change – you lose some of the impatience and I can take the long view now."

So what about that phrase: "it's just in the DNA"? There seems to be some truth in it –Dunion comes from a family of solid Labour voters– but as we talk, other possible sources for his radicalism start to emerge: his days at university as a campaigner; the time in India he was chased through the mountains by the police; or even the moment he saw for the first time a picture of the Earth from space.

"When I was 12 nobody on the planet had seen that picture," he says. "The big, fragile Earth." It's an image that seems to have been with him when he became chief executive of Friends of the Earth Scotland.

As for the DNA, it turns out one of its strands is radical but another is Italian. Dunion's maternal grandparents were Italian and had shops and restaurants and an ice-cream parlour in Alloa where he grew up. They lived just across the road from his other grandparents, all close and familiar like the grannies and grandads in Charlie And The Chocolate Factory. "That was how I thought it should be," he says. "I could visit them both and get sixpence from them both, all on the same Sunday."

Sadly, the old ice cream parlour is gone, but Dunion remembers the shop they had in Tillicoutry. When he was nine, he would work behind the counter. "You wouldn't be allowed to do it now – I used to get the bus from Alloa, serve in the shop, sell cigarettes to the mill girls, and I loved it."

Later, when Dunion went to St Andrews University to study economics, some of the frustrations that have guided his career started to emerge. This was the seventies and his lecturers were telling him you couldn't have high unemployment and high inflation. "But of course you just had to walk outside to see that was exactly what we were experiencing," says Dunion, "so I got impatient with being taught economics that didn't relate to the real world."

The other issues motivating Dunion then were environmentalism, at a time when not many people were talking about it, and devolution, at a time when everyone was talking about it. He knew Alex Salmond, who was also a student at St Andrews, and campaigned for a yes vote in the 1979 referendum. And so where does he stand on independence? He smiles ruefully. "I'm still information commissioner so I can't express an opinion," he says.

After university, Dunion's first job was ill-starred: working for the Inland Revenue in Arbroath scrutinising fishermen's tax returns. It clearly wasn't want he wanted to do – that was politics with a small p and so he left to work for Edinburgh University's Student Association and campaigned for students' interests.

After that, he joined Oxfam as a campaigner and it was here the troublemaker really got going. He spent two months in India, travelling from north to south, and it was another of the seminal moments in his career. Here, as he saw it, was a medieval agricultural population being abused by a thrusting industrial one and Dunion's target became India's massive dam-building project. "Ways of life were being destroyed," he says.

Dunion's response, even though it was illegal, was to drive up to the Narmada dam in Gujarat and take photographs as evidence of what was happening. The police spotted him and he was chased over the hills. Later, the pictures were used as the basis of a television documentary.

After seven years with Oxfam, Dunion became chief executive of Friends of the Earth Scotland and one of the first things he did was test whether public authorities would respond to requests for information – which led to him becoming the individual most associated with the FOI campaign.

This probably meant he was the logical choice to be commissioner when the freedom of information legislation was passed in 2002. The post comes with an annual salary of £85,000 and a beautiful office, so did Dunion worry that, after years of pushing FOI as an applicant, he was in some ways changing sides?

"I can give you the exact rationale for it – I was delighted we had the legislation. And it became clear this role of the commissioner was massively important in terms of how the law was going to be interpreted. And so I thought: why not?

"As to whether I'm part of the establishment, of course inevitably you are – I've got the Queen's Commission sitting on the wall behind me there – but I think I've taken progressive decisions. I think my decisions stand up to scrutiny. I could have been more cautious but the whole point is to call it as you see it and not be afraid of upsetting the authorities."

He has received a lot of opposition from those authorities over the years, some of it ludicrous. One police force, for example, argued that releasing the price of a police Range Rover would lead to the collapse of the emergency services. His response has always been simple: I don't believe you.

Some decisions have been harder than others – in the early years, he received many requests relating to Dunblane but when he went to the police station and looked through the files, he decided he would employ an unwritten exemption of gross public distaste. He's sometimes had to do this: rely on his sense, his instinct, a feeling for what's right.

He certainly doesn't feel he's taken risks with privacy and is proud of his progress. He believes the release of commercial contracts, such as PFI contracts for Edinburgh Royal Infirmary, have changed the way business is done, but he also thinks the culture of Scotland has changed. We are more open, he says, although the battle is ongoing.

Which leads us, in Dunion's last week as commissioner, to his final warning. When Tony Blair left office, he said the FOI legislation was one of his biggest regrets and Dunion says this danger will always exist – that in the end, the authorities will regret being open and try to resist. It's a danger we must fight, he insists. We had to struggle to get freedom of information. Now we must struggle to keep it.

Favourite food?

Scottish scallops; Italian risotto.

Favourite holiday destination?

South Island, New Zealand.

Favourite film?

In Bruges.

Favourite music?

Most played on my iPod would be Neneh Cherry, Eels, Goldfrapp, Steely Dan and St Germain.

Last book read?

Into The Silence – Wade Davis brilliantly provides the geo-political context of Mallory's attempts to climb Everest.

Biggest influence on you?

Oxfam – inspirational people; life-changing experiences.

Perfect dinner guests?

For an evening of gossip and wicked humour, I'd love to gather Armando Ianucci (pictured), Kirsty Young, Billy Connolly and Anna Netrebko.

Best advice received:

Be radical enough to make a real difference, and be pragmatic enough to make it happen.

kevin dunion

LIFE AND LOVES