The UK Government is to invest £120 million in a support package for the UK's last ethylene plant in Grangemouth.
Around 500 people work at the site, which produces plastics.
It means the site will remain open.
Ineos will contribute £30 million as part of the joint investment deal.
Prime Minister Sir Keir Starmer said: "We're delivering new opportunities, fresh investment and security for the next generation of workers in Scotland.
"Our commitment is clear; to back British industry, to stand by hardworking families, and to ensure places like Grangemouth can thrive for years to come."
Ineos owner Sir Jim Ratcliffe said:
"Through this partnership, Ineos and the UK government have demonstrated their commitment to operating the site and maintaining jobs. The agreement includes safeguards to protect taxpayers’ money and ensures the funding is used to strengthen the plant’s long-term future."
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