The effects of the kind and amount of oxirane (propylene or ethylene oxide), the amount of triethylamine catalyst, and the temperature on the reaction with N,N,N’,N’,N "-pentakis (hydroxymethyl)melamine (PHMM) in DMSO solution were studied. The changes in the content of reactive groups in the reaction mixture and in the final product were analyzed. The oxirane addition was shown to be accompanied by the condensation of hydroxymethyl groups in the PHMM. Optimal conditions for the synthesis and the structural formula of the s-triazine ring containing polyetherols were established.