The hydroxymethylation reaction of urea, monomethylolurea, N,N-diethylurea, acrylamide, two derivatives of tetrahydro-1,3,5-triazine-2(1H)-one and melamine in aqueous solutions (phosphate buffer, pH 8.0) at 30 degrees C has been studied using a Calvet-type microcalorimeter. The values of the heat effects and rate constants for the formation of hydroxymethyl groups have been determined.