Unite says that its members wanted the union to work with Ineos to implement its proposals:
Scotland's First Minister Alex Salmond said: "This news is a tremendous fillip for the workforce and the whole Grangemouth community, following what could have been a potential disaster.
"It's been a great team effort from all concerned, including the unions and workforce, the management, Governments - and BP, who have made a material contribution to help defend and secure Scottish jobs and livelihoods.
"I am delighted that people have rallied round to protect these jobs, and now we can all agree that Grangemouth has an outstanding future."
This is a big capitulation by the union to save 800 jobs at the Grangemouth plant.
Pay freeze, no strikes and a pensions cut. This is the "something big" that Ineos demanded.
The owners of the Grangemouth site in Scotland will open the petrochemical plant and the oil refinery immediately after both were shut down last week over an industrial dispute.
The move follows acceptance by the Unite union of a survival plan, including a pay freeze, ending of the final salary pension scheme and other changes to terms and conditions.
An undisclosed number of contractors have been laid off or switched to other sites.
Asked what the reaction from workers had been, Calum McLean said that the news came as a "great relief to the site".
Calum McLean, chairman of Ineos Grangemouth, has said that there is a 15-20 year future for the petrochemical plant.
He said there had been a "change of mind" by the Unite union and that the firm had received a "very clear message from the employees".
He added that the company plans to invest a further £300 million in the plant and that "redundancies will be very limited".
Ineos has decided to keep its Grangemouth petrochemical site open after Unite accepted a survival plan, PA sources said today.
The owner of the Grangemouth refinery, Ineos, has reversed an earlier decision to close the petrochemical plant saving around 800 jobs, according to media reports.
BBC News reports that the union may have to make further concessions to Ineos.
Workers at the troubled Grangemouth plant in Scotland have been invited to a meeting at 11am, Sky News reports.
Staff at the site are waiting to hear if the firm will reverse its decision to close part of the plant after the Unite union accepted a survival plan aimed at securing the site's future.
The owner of the Grangemouth plant is expected to announce whether it will reverse a shock decision to close the site, safeguarding hundreds of jobs.
Ineos has been discussing a change in position by the Unite union whose members now say they will commit to a plan aimed at securing its future.
The petrochemicals plant and adjoining oil refinery making up Scotland's largest industrial complex was shut down last week in advance of a planned walkout over pay and conditions.
Ineos did not restart the site after Unite called off the strike but wrote to staff asking them to sign up to changes such as a pay freeze and the closure of the final salary pension.
The company insisted on Wednesday it had no alternative but to close the plant after it failed to persuade its staff to accept the survival plan but Unite general secretary Len McCluskey said yesterday the union would embrace it "warts and all".