WASHINGTON — The Supreme Court on Monday ruled that a chemical company may be able to cut the health benefits of its retired workers, unanimously reversing an appeals court ruling that said the benefits had vested for life.

“Courts should not construe ambiguous writings to create lifetime promises,” Justice Clarence Thomas wrote for the court, adding that “retiree health care benefits are not a form of deferred compensation.”

The Supreme Court returned the case to the appeals court, telling it to use ordinary principles of contract interpretation to determine whether the collective bargaining agreement at issue had granted free lifetime health care.

The appeals court erred, Justice Thomas wrote, “by placing a thumb on the scale in favor of vested retiree benefits in all collective bargaining agreements.”