Thermal dehydration of a poly(vinyl alcohol) (PVA) film was greatly promoted by diphenyliodonium trifluoromethanesulfonate, which is known as a thermally stable photo acid generator. The oxidation of thermally generated PVA radicals by the salt is considered to lead to the dehydration. Liquid-nitrogen trapping of volatile products during the heating largely accelerated the dehydration for the salt-free film but not for the salt-containing film.