In a process for preparing carbonyldiimidazole of the general formulae Ia, Ib, Ic or mixtures thereof where R1 is hydrogen or a C1-4-alkyl radical and R2 is hydrogen or a methyl radical, by reacting at least one imidazole of the general formulae IIa and IIb in which R1 and R2 have the abovementioned meaning, with phosgene in an inert solvent, stoichiometric amounts, based on the imidazoles, of an organic nitrogen base which has a lower pKb than imidazole are added to the reaction mixture.