Unite said it was taking tribunal proceedings against the Scottish site's owners Ineos on behalf of Mark Lyon.
The tribunal is being asked to find that he was unfairly dismissed for reasons relating to the representation he provided as a trade union convenor.
Unite regional secretary Pat Rafferty said "Our members in Ineos have lost a valued colleague and representative in Mark Lyon and the allegations about his treatment by the company are now before an employment tribunal.
"Despite Ineos' claims of wanting to draw a line under matters at the Grangemouth complex, it recently cancelled arrangements to pay union subscriptions from Unite members' wages.
"Such negative action by the company has not deterred the workforce from continuing their membership of Unite via direct debit in their hundreds as they seek to regroup and move on."
Another Unite convenor at Grangemouth, Stevie Deans, resigned last year.
Ineos threatened to close part of the site but changed its mind after workers accepted changes to pay and conditions.