Libya has declared a 20-ton stockpile of deadly mustard gas in a full report on its chemical weapons programmes submitted to the UN on Friday.
Gaddafi has already ordered the dismantling its weapons programme
The Libyans also detailed large amounts of chemicals used to make nerve gas.
The UN says the declaration is a major step towards eliminating Libya's weapons of mass destruction.
It follows Libya's surprise announcement in December that it was scrapping its weapons programmes, in a bid to end its international isolation.
"It's a great day," said Rogelio Pfirter, director-general of the Organisation for the Prohibition of Chemical Weapons (OPCW), after receiving Libya's documents at the OPCW's headquarters in The Hague.
"This is good for Libya, the region and the international community since it strengthens this multilateral disarmament regime and represents a tangible step towards the ultimate elimination of these weapons of mass destruction," Mr Pfirter said.
As well as mustard gas, the report discloses that Libya possessed large amounts of chemical agents used in the manufacture of sarin and other toxins.
These stocks will all have to be destroyed under the watchful eye of inspectors.
The UN has already praised Libya after it supervised the destruction this week of more than 3,000 empty chemical bombs.
The BBC defence correspondent Jonathan Marcus says it was a decidedly low-tech affair - bulldozers were used to crush the casings of the unfilled chemical bombs.
The Libyan example is an unusual one in the world of arms control, our correspondent says.
Last December, the Libyans revealed their intention to give up all weapons of mass destruction, following months of secret talks with British and US officials.
In response, Washington last month lifted a ban on its citizens travelling to Libya after 23 years of wide-ranging sanctions.
Most trade restrictions still remain but US companies are now allowed to prepare for a return to Libya.
The US decision also followed Tripoli's clarification that it accepts responsibility for the Lockerbie bombing in 1988, over-riding comments by the prime minister who denied Libya had any part in the attack.
Nuclear inspectors from the OPCW's sister organisation, the International Atomic Energy Agency (IAEA), have already been at work assessing Libya's nuclear weapons programme, although this was less advanced than the country's chemical weapons activities.