As part of our study on the curing reactions between novolac resins and hexamethylenetetramine (HMTA), a model benzoxazine, 3-(3,5-dimethyl-2-hydroxybenzyl)-6,8-dimethyl-3,4-dihydro-(2H)-1,3-benzoxazine, was heated under carefully controlled conditions, and the structural changes were studied by C-13 and N-15 n.m.r. spectroscopy. The benzoxazine structure is relatively stable, and detectable decomposition only occurred about 185 degrees C with the formation of methylene linkages between phenolic rings. Various nitrogen-containing structures, such as amides, amines and imines, together with hydroxybenzyl alcohol, bis(ortho-hydroxybenzyl) ether, hydroxybenzaldehyde and ortho-hydroxybenzoic acid, are also formed as the side-products of the decomposition. The dominant product after heating the sample to 240 degrees C is 2,2'-methylene-4,4',6,6'-tetramethyldiphenol. These findings when applied to novolac/HMTA systems provide an explanation for reaction mechanisms/reaction pathways from the benzoxazine intermediates to the final cross-linking network. (C) 1997 Elsevier Science Ltd.