A process is described for preparing diacetals of glyoxal by reacting from 40 to 75% by weight aqueous glyoxal with monohydric alcohols in the presence of an acid catalyst, which comprises leaving a liquid mixture which, at the beginning of the reaction, comprises alcohol and glyoxal in a molar ratio of at least 15:1 and also water in a concentration of not more than 8% by weight in contact with the acid catalyst until concentration in the reaction mixture of the diacetal formed reaches at least 70% of the equilibrium concentration without more than 5% by weight of the alcohol used having already been distilled off.