PURPOSE: To obtain a primary organic amine easily in high yields and purity by reacting a specified isocyanato-containing organic isocyanate with formic acid to formylate the isocyanate groups and converting the N-formyl groups into amino groups.

CONSTITUTION: An organic isocyanate containing aliphatically or cycloaliphatically bound isocyanate groups (e.g. hexamethylene diisocyanate trimer) is reacted with formic anhydride in a solvent such at toluene at 20-150°C to formylate the isocyanate groups into N-formyl groups, and the product in subjected to a formyl exchange reaction with a primary or secondary amine having a boiling point of 80-150°C (e.g. ethylenediamine) to convert the N-formyl groups into amino groups to obtain the objective compound such as a polyamine mixture based on N,N',N"-tris-(6-aminohexyl)isocyanurate and its higher homologue and being useful as a hardener for epoxy- or isocyanato-containing polymers.