Unite says Mark Lyon was dismissed for not stopping the union from commenting over fears of job losses at the plant. Now the union says it has started employment tribunal proceedings on behalf of Mr Lyon against owners Ineos.
A tribunal is being asked to find that Mark Lyon was unfairly dismissed for reasons relating to the representation he provided as a trade union convenor.
The union says Ineos convened a disciplinary hearing despite Mr Lyon being off work with stress.
The union is taking the step of asking the tribunal to make an interim finding in Mr Lyon's favour in order to avoid the financial hardship he will face between now and the final hearing.
Ineos last year threatened to close part of the site if workers did not agree to a rescue package to help secure its future.
An Ineos spokesman said: "A full and thorough investigation into his [Mark Lyon's] behaviour and activities whilst employed at Grangemouth has been reviewed prior to taking this decision. Company disciplinary policies were applied as per normal procedure."