A process for the preparation of 1-(pyrimidin-2-yl)propan-2-ones of general formula (I), in which R is in each case a C1-10-alkyl group, a C3-8-cycloalkyl group, an alkyl group or an aryl-C1-4-alkyl group. For this, a malondiimidate of the general formula (II), in which R has the meaning given above, is reacted with diketene. The compounds which can be prepared according to the invention are intermediates for the synthesis of agrochemical active ingredients.