Polyamines containing aromatically bound primary amino groups are prepared in a single stage process by hydrolyzing an isocyanate in the presence of a compound containing an amidine and/or guanidine group and a water-miscible, aprotic-dipolar solvent. This hydrolysis is carried out at from 20.degree. to 210.degree. C. with the reactants being in substantially homogeneous phase. The isocyanate starting material contains aromatically bound NCO groups and has NCO content of from 0.5 to 40 wt. %. The compound containing amidine and/or guanidine groups functions as a catalyst and is used in a quantity of at least 0.001 parts by weight for every 100 parts by weight of isocyanate. The water-miscible solvent is used in a quantity of at least 10 parts by weight for every 100 parts by weight of polyisocyanate. The polyamines thus-produced are particularly useful in the production of polyurethanes.