A team from the Organisation for the Prohibition of Chemical Weapons is inspecting a Defence Research and Development Organisation laboratory making chemical weapons at Ozra near the western town of Nashik.
The preliminary inspection, covering only chemical weapons production sites, comes three years after India signed the Chemical Weapons Convention. It ratified the treaty in September.
The 10-member inspection group is the second to visit India after a four-man team from the same organisation inspected another defence research chemical weapons facility at Gwalior, west of New Delhi, in July.
Having signed the convention, India has given the Organisation for the Prohibition of Chemical Weapons details of all sites associated with such weapons, enabling its inspectors to select laboratories, storage sites and items for scrutiny.
The inspectors, who are accompanied by a defence official, can also photograph the sites and ask for all documentation and records pertaining to chemical weapons.
India has also established a national authority to collaborate with the anti-chemical weapons group and train inspectors and industry personnel to carry out the convention's aims of destroying all chemical weapons and closing production facilities by 2007.
Although details of India's chemical weapons stockpile are limited to a handful of senior defence research officials, experts claim it used binary technology to fuse some highly effective nerve gases developed during World War II.
Experts said the chemical weapons reportedly contained two safe chemicals which, mixed just before detonation, produce a gas whose lethal impact was second only to nuclear weapons.
The Chemical Weapons Convention came into force in April after 78 members including China and the United States ratified it. It is the first global regime to target an entire category of weapons of mass destruction with a view to eliminating them.
Analysts expressed surprise that, despite repeated denials, India admitted in June that it had an arsenal of chemical weapons and the capability to produce them.
But they said declared possession of chemical weapons would act as a deterrent, indicating to hostile neighbours such as Pakistan and China that India's posture on weapons of mass destruction was not a bluff but a reality.