In order to prepare polyester or polyetherpolyester polyols, polyols, preferably di- to hexafunctional polyether polyols having hydroxyl numbers from 15 to 250 are esterified with at least one carboxylic acid anhydride, preferably phthalic acid anhydride, in the presence of N-methylimidazole, triethylenediamine, and/or triphenylphosphine as catalysts to form a carboxylic acid half-ester, and said half-ester is then oxyalkylated with at least one alkylene oxide, preferably ethylene oxide, in the presence of N-methylimidazole, triethylenediamine, triphenylphosphine, thiodialkylene glycol, employing a mixture of at least two of the compounds cited as catalysts.