The Unite union said Ineos had drawn up plans to close five sections at Grangemouth, four at the petrochemical operation and one at the oil refinery, with workers being "sacked on the cheap".
Staff had been expected to be told of the moves yesterday but Unite said that by last night no-one had been informed.
Shift payments will be cut by £2500 a year and new employees will receive lower wages.
Swiss-based Ineos was reported to have said the firm would issue a full statement after employees had been told the news. However, no-one from the firm was available for comment last night.
The company had threatened to close the petrochemical site in October with the loss of more than 800 jobs unless workers accepted changes to pay, pensions and terms and conditions.
Unite's Scottish Secretary, Pat Rafferty said: "Loyal staff are paying a huge price with their livelihoods and terms and conditions to save Grangemouth. They deserve answers, as do taxpayers who are ploughing in millions of public funds in their belief that Ineos's survival plan is about jobs.
"Yet as the consultation over that plan comes to an end it is becoming apparent that part of the massive price workers will pay includes huge cuts in redundancy terms that come into effect from January 1, 2014.
"Many will view this as a move by Ineos to sack loyal workers on the cheap. Ineos needs to come clean over job losses, the timescales involved and provide assurances to the workforce this Christmas."
Scottish Labour finance spokesman Iain Gray described the prospect of job losses at Grangemouth as deeply regrettable.
He added: "We must ensure a sustainable future for Grangemouth is found and support is put in place for those workers who do lose their jobs."
Grangemouth employs nearly 1400 permanent staff as well as about 2000 contract workers.
The recent stand-off between Ineos and Unite, which represents many of the workers at Grangemouth, nearly resulted in the plant's closure and the loss of more than 1000 jobs.
Initially triggered by the company's investigation of shop steward Stevie Deans, who was also at the centre of the Labour selection debacle in Falkirk, the dispute snowballed and led to the temporary shutdown of the site.
Mr Deans, who was accused of using company time to conduct Labour Party business, later resigned.
Workers at the petrochemical site, and adjoining oil refinery, had refused to sign up to a survival plan, which included a pay freeze, ending of the final salary pension scheme, and other changes to terms and conditions.
Ineos then threatened to close the petrochemicals side of the Grangemouth plant, sparking political concern both at the Scottish and Westminster Government and a climb down by Unite.
Ineos has repeatedly claimed the plant is losing money. It is, in fact, profitable, but is paying out more cash than it is taking in because of investment commitments. The Scottish Government said it would be "working to minimise the impact of any job losses".