Multiple injections were given to troops
However, a UK defence minister has insisted that the ingredient, squalene, was never given to British personnel in vaccinations.
Research says veterans of the 1991 conflict appear more likely to suffer a variety of severe and long-lasting illnesses than soldiers who served elsewhere.
Many believe that the different combinations of symptoms, such as severe headaches, short-term memory loss, extreme fatigue and aching joints, are manifestations of one illness - "Gulf War syndrome".
However, despite theories about the origin of these symptoms, no single cause has ever been established.
And some doctors hotly dispute the idea that these different illnesses could be related to a single cause.
All had been given injections against possible germ warfare threats such as anthrax.
The scientists were looking for signs that the body's immune system had come into contact with a chemical called squalene.
It can be added to vaccines to increase their potency.
They found that 95% of the affected veterans had antibodies to squalene.
A small number of personnel who were given the vaccinations, but who never went to the Gulf, also had the antibodies.
While the findings suggest a strong link between an immune reaction to squalene, they do not prove exactly how this has happened - and the source of the squalene itself.
However, Dr Pamela Asa, who led the research, said that the findings provided firm evidence that the vaccines were culpable in Gulf War syndrome.
Animal disease
Squalene is a chemical which occurs naturally in the body, but it is not licensed for medical use in the UK.
In animal experiments, it can cause a form of auto-immune disease.
Tony Flint, from the Gulf War Veterans and Families Association, told the BBC: "We've known about squalene for quite a long time. British Gulf veterans have been tested positive for squalene in their systems."
He said that he did not expect the government to welcome the new research.
"As long as it's not their research, they deny it exists.
"They did not know what they were doing with the vaccines. They just don't want to admit liability."
Dr Lewis Moonie told the BBC that a testing programme that testing might reveal the antibodies for squalene in veterans.
But he said: "I can give a categorical assurance that they didn't get it from any vaccination.
"There's no such thing as Gulf War Syndrome - there's a collection of illnesses which don't fit any particular pattern."
The government is funding its own ongoing research into Gulf War illnesses.
A study published in the British Medical Journal just over a year ago pointed to a link between later illness and multiple vaccinations given to soldiers after they arrived in the Gulf.
However, even this was deemed "inconclusive" by other experts.