The present invention relates to a process for purifying crude polymethylol which comprises polymethylol of the formula (I) (HOCH2)2—C—R2  (I) in which each R is independently a further methylol group or an alkyl group having 1 to 22 carbon atoms or an aryl or aralkyl group having 6 to 22 carbon atoms, and also hydroxy acid of the formula (IV) in which each R is independently as defined above, which comprises performing the purification in a distillation column, the bottom of the distillation column being connected to an evaporator with a short residence time. The present invention further relates to a composition comprising polymethylol and 1 to 10 000 ppm by weight of an ester of polymethylol and of a hydroxy acid and to the use thereof.