Disclosed is a method for the trimerization of organic isocyanates comprising thermally activating, in the presence of said isocyanates, a catalyst comprising a carboxylic acid salt selected from the alkali metal salts or quaternary ammonium salts of particular substituted malonic acids or half-esters thereof, particular carbamyl substituted aliphatic acids, or triaryl acetic acids.Mixtures of the catalysts with isocyanates have surprisingly long periods of stability at ambient temperatures (about 20.degree. C.) which makes the present method particularly suitable to systems for molding polyiso-cyanurates wherein long open times are required of a resin pre-mix prior to the actual curing step.