A process for the preparation of polybutylene glycol carboxylic acid diesters of the formulaR--CO--O--CH.sub.2 CH.sub.2 CH.sub.2 CH.sub.2 --O).sub.n CO--R.sup.1by polymerizing pretreated tetrahydrofuran in the presence of one or more carboxylic acids and/or carboxylic acid anhydrides and of a polymerization catalyst, especially antimony pentachloride or a bleaching earth containing less than 3% by weight of water, wherein the tetrahydrofuran is treated with a strong mineral acid, an organic sulfonic acid, silica gel and/or a bleaching earth prior to the polymerization.