The instant invention is directed to a process for the production of heat-resistant synthetic resins containing isocyanurate groups by the polymerization of organic polyisocyanates in the presence of catalysts which accelerate the trimerization of isocyanate groups, wherein said polyisocyanate component is a mixture of isomers and/or homologs of polyisocyanates of the diphenylmethane series containing more than 20%, by weight, of 2,4'-diisocyanatodiphenylmethane.