Although we are dependent on the stuff every moment of every day, few of us give much thought to oxygen - let alone to the question of who discovered it. And yet it is hard to over-estimate the importance of that discovery. Not only did the man in question contribute indirectly to our ability to fly to the moon and to the development of modern medicine, but his realisation that air was not one element, but several, marked the beginning of all modern chemistry.

Give that man a medal, you might say. But not so fast. Which man would you give it to? This is the question that preoccupies the characters of a new play, Oxygen, which opens at London's Riverside Studios next week. Charged with awarding the first "retro" Nobel Prize for chemistry, they soon realise that they have three contenders in the frame. Should they honour Wilhelm Scheele, who made it first; Joseph Priestley, who published first; or Antoine Lavoisier, who first understood its significance?

The play raises fundamental questions about the nature of discovery. Who discovers something after all - the person who finds it, or the one who understands it? And why does it matter who gets there first? The action shuttles back and forth between 1777 and 2001, contrasting the fierce competition in the past with the fiery arguments in the present, and blowing sky high the illusion that there was once a golden age of science unpolluted by base motives such as the desire for fame. They play was written by Carl Djerassi and Roald Hoffmann, both distinguished chemists and authors. Given the play's emphasis on creative competition, one has to ask the question: who had the idea first?

Sitting side by side on the sofa in Djerassi's London flat, the two men look at one another and laugh. Appropriately, or perhaps diplomatically, they cannot remember. Like the scientists in the play, they approached the idea simultaneously, from different directions. Unlike the scientists, they decided to pool their resources. This was not an easy option: "Neither of us realised how few plays are written by two people," recalls Djerassi. "And there is a very good reason for that."

The two men have much in common. They were both European refugees (Djerassi from Austria, Hoffmann from Poland) who were educated in the US; they both combine an award-laden career in chemistry with writing (Hoffmann poetry, Djerassi several novels and a play). Yet their differences are what strike you. Djerassi has a restless intelligence, sits upright, and speaks in rapid, inflected English; Hoffmann appears more mellow, reclines back into the sofa and has a soft-spoken American accent. Long-standing friends and colleagues, they are courteous and good-humoured. But as they jostle for the subject matter, they sound rather like a scene from the play.

Djerassi: We originally envisaged taking a character each. We thought we would sit down and write together, one taking one voice and the other taking another.

Hoffmann: We also had a naive notion that we could write it in real time. We tried. We wrote the first scene -

Djerassi: - together in a hotel in New York. Originally we wanted a prologue with the two characters Hoffmann and Djerassi. Pretty soon we learned that that was not what we wanted to do.

Hoffmann: It didn't work. I wonder how Fletcher and Beaumont managed.

Djerassi: Well, they had the great advantage of being in the same place at the same time. I was in San Francisco and London and Roald was in Ithaca. Without e-mail it couldn't have been done.

It's easy to see how writing in the same room might lead to friction. The attempt was not wasted, however - they did in fact write some of their disagreements into the play itself. Eventually they settled on a modus operandi that made the best of their complementary styles: they worked simultaneously, but apart, zapping versions of scenes back and forth through cyberspace.

As scientists, they were probably better equipped than most writers to cope with joint authorship, being accustomed to collaboration on papers.

"Science is the most collegiate of the creative enterprises and yet the most brutally competitive," says Djerassi. "In science when you collaborate on a paper, usually there is someone who is the senior author and everyone recognises that: it's a pyramid. But in this case we have two pyramids of exactly the same height - and you can't put one on top of the other."

They pre-empted conflict by drawing up what they affectionately refer to as a "prenuptial agreement". They also signed up a dramaturg to act as arbitrator if they couldn't agree. The very process of writing the play, then, was in keeping with its preoccupation with the ownership of ideas and the role of authorship. At one point in the play, Priestley complains that Lavoisier has spoken of experiments using the pronoun "we" to evade the question of who did it first. This is a matter close to the hearts of Hoffmann and Djerassi, both of whom rel ish the stylistic freedom that drama allows. The detached voice of a scientific paper "automatically goes for a consensus", says Hoffmann, whereas the very nature of drama means that it is able to present multiple, even contradictory viewpoints.

"Drama allows for the inherent greyness of issues to be expressed," agrees Djerassi. He adds that after years of following the "tribal custom" when writing his scientific papers, being able to write dialogue came as a breath of fresh air: "We scientists are not permitted to use dialogue. So a large proportion of my fiction is dialogue - it releases the 40 years of not realising how much I wanted to do it."

This contrast in intellectual approach might explain why so few chemists take to the stage. The finished drama joins a distinguished, but rather short, list of plays about science - and an even shorter one of those written by scientists.

"There are relatively few serious plays about science," says Djerassi. "And all of those playwrights, starting with Brecht and ending up with Michael Frayn, have learned their science second hand. They are sponges - and I mean that in a positive rather than pejorative sense - with the ability to assimilate material and translate it into marvellous language and marvellous settings. But we come from a completely different direction.

"Some very well known physicians have written plays - Chekhov and Schnitzler for example - but I know really of no chemists who have written a play. And I believe that the reason is that we chemists are so involved in abstraction."

As chemists who write fiction, the two men find themselves regarded warily both by the science-shy public and by their colleagues. But Djerassi in particular is proud of his ability to smuggle science into fiction and to demystify it for the public. And audiences for the play will certainly emerge wiser about early experiments and about the unsung role of women in scientific endeavour. But will they find out who discovered oxygen?

"The question people always ask is, 'In your opinion, who would have got the prize?'" says Djerassi. "And we tell them that we actually argued for hours over it. But even if we did agree we wouldn't give the answer because it would have no meaning. The most important theme in our play is 'What is the nature of discovery?' And that has not been resolved." 'Oxygen' opens at the Riverside Studios, London W8 on Wednesday; a radio version is broadcast on BBC World Service on December 1 and 2.

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