The present invention relates to a process for the preparation of aliphatic, cycloaliphatic, araliphatic and aromatic mono- and oligoisocyanates by phosgenation of the corresponding primary amines at atmospheric pressure, which comprises a) introducing a catalytic amount of a monoisocyanate (isocyanate a) into an inert solvent together with phosgene; b) adding the primary amine, and c) reacting the resulting reaction mixture with phosgene.