The U.S. National Labor Relations Board (NLRB) said a 10-month lockout of workers at an Exxon Mobil Corp. refinery in Texas was an “unlawful” effort to remove the United Steelworkers union (USW) representing the workers, according to a complaint issued on Oct. 3.

The NLRB asked an administrative law judge to issue back pay, among other remedies, to the more than 600 workers locked out of their jobs at Exxon Mobil’s Beaumont, Texas, refinery and lube oil plant between May 2021 and March 2022.

A hearing on the complaint and proposed remedies is scheduled for January in Houston. Union refinery workers with at least four years experience nationally make over $41 per hour on average. The back pay could cost Exxon Mobil tens of millions of dollars.

“Exxon Mobil acted in accordance with the law at all times,” spokesperson Julie King said in a statement.

Exxon Mobil issued messages to the locked-out workers promising they could return to their jobs if they voted to decertify USW local 13-243 as their representative, the NLRB said.

“If [Exxon’s] lockout had been lawful, by its conduct [seeking decertification], the lockout was converted to an unlawful one,” the NLRB said.

Decertification would have removed the union from representing the workers in the refinery. However, workers voted on March 14 to keep USW local 13-243 as their representative.

Exxon Mobil “provided more than ministerial aid to efforts of employees in their attempts to decertify” the union, according to the NLRB notice.

Bryan Gross, a USW International representative, said the board’s complaint was welcome news for workers locked out by Exxon Mobil.

“We’re glad the board saw what we saw,” Gross said on Oct. 3. “We stood by our members.”

Exxon Mobil has said it began the lockout in response to a strike notice issued by the union during negotiations in January 2021 for a new contract. A lockout was necessary to prevent potential disruption to the 369,024 bbl/d processing facility.

The refinery, Exxon Mobil’s third largest in the U.S. by capacity, remained in operation throughout the lockout with replacements hired locally or transferred in from other Exxon Mobil plants.

The NLRB said the use of replacements was “inherently destructive of the rights guaranteed employees” by federal law.