Process for preparing methyl formate by carbonylation of methanol by means of carbon monoxide in a carbonylation reactor in the presence of a catalyst system comprising alkali metal formate and alkali metal alkoxide to give a reaction mixture (RM) which comprises methyl formate, alkali metal formate, alkali metal alkoxide and possibly unreacted methanol and unreacted carbon monoxide and is taken from the carbonylation reactor, wherein the reaction mixture (RM) comprises at least 0.5% by weight of alkali metal alkoxide based on the total weight of the reaction mixture (RM) and the molar ratio of alkali metal formate to alkali metal alkoxide in the reaction mixture (RM) is greater than 1.