Combined experimental and computational studies have been performed on the mechanism of formation of poly(hexahydrotriazine) and hemiaminal dynamic covalent network (PHT and HDCN) thermosetting polymers from the reactions of diamines with formaldehyde (Science 2014, 344, 732-735). Results suggest that these polymers are formed by a mechanism involving the water promoted stepwise addition of amines with formaldehyde in preference to dimerization or cyclotrimerization of imine intermediates or self-catalysis by the amine reagents. The predicted mechanism also explains experimentally observed electronic effects for hexahydrotriazine formation.