An improved process for preparing a nitrogen or oxygen containing organoalkoxysilane such as an isocyanurate or fumarate by reacting a haloalkylaikoxysilane such as a chloropropytrimethoxysilane and a reactant of either a cyanate or an ammonium salt of an organic acid of C1-C20 carbon atoms or a metal salt of an organic acid of C1-C20 carbon atoms or mixtures thereof to a reaction temperature of at least about 80.degree. C. to about 200.degree. C., the improvement is in carrying out the reaction in the presence of a guanidinium salt phase as a transfer catalyst such as hexaethylguanidinium chloride. The improved process also is reacted wherein the temperature increase from the exotherm reaction above the reaction temperature ranges from about 0.degree. C. to about 120.degree. C. A preferred metal cyanate is sodium cyanate.