Chemical giant Dow is set to close part of its Barry works, with basic siloxane operations ending by the middle of 2026.
It's understood that the works will continue to produce downstream silicones.
The news comes as Dow announces wider plans to shut down three major upstream facilities in Europe - including two in Germany - with the loss of around 800 jobs.
291 jobs were initially expected to go locally, but the number of expected redundancies has been reduced.
The Unite union says it expects 200 job losses following talks with Dow - with nearly 150 of these expected to be voluntary redundancies. Employees have also secured "enhanced" redundancy packages.
Unite Regional officer Anthony Simpson said: "Unite’s hard work in negotiations has managed to save some of the jobs at risk and ensure that those who are being made redundant receive enhanced packages."
"However, going forward we need to ensure there is government support for retraining across the board. We need a proactive industrial strategy that underpins our manufacturing industry to ensure there is decent future employment in the Barry area."
Dow has operated from the Barry site since 1971, when it bought the site from Midland Silcones, and now employs 850 people across a 160-acre site.
The firm previously revealed that cheap competition from abroad was behind the potential closures as its basic products are no longer competitive on the open market, with China undercutting prices.
“Our industry in Europe continues to face difficult market dynamics, as well as a continuing challenging cost and demand landscape,” said Dow chief executive Jim Fitterling.
"Over the past decade, we have demonstrated Dow's commitment to operating with a best-owner mindset by taking proactive actions across higher-cost or non-strategic assets."
"Looking ahead, we remain committed to realizing the value of our incremental growth investments and enhancing profitability and cash flow through more than $6 billion in near-term cash support."
Dow said the phased shutdowns, scheduled to begin in mid-2026 and completed by the end of 2027, will look to eliminate higher-cost, energy-intensive operations from its European portfolio.
But decommissioning and demolition activities are expected to extend into 2029 - with the Unite union saying a consultation is ongoing.
A three-month consultation with staff at the Barry works began in April - with plans to close the basics area rubberstamped last week by Dow's board of directors.
Unite general secretary Sharon Graham said: "Dow's callous decision to cut jobs is devastating to both our members and their families and the local economy."
"Our reps have worked hard to mitigate the impact of redundancies and we will fight every step of the way to ensure as many members as possible are retrained and moved into job roles in other areas of Dow."
South Wales Central MS Andrew RT Davies described the news as a "devastating blow to the town."
The former Welsh Conservative leader said: "These are high quality, skilled jobs – and this news is a devastating blow to Barry and the Vale of Glamorgan."
"Yet again, our communities are suffering the consequences of globalisation – with local people paying the price."
"This, combined with an endless supply of unsustainably cheap labour from abroad – is making it increasingly hard to earn a living and provide for families. Our economy needs a reset urgently."
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