The reaction between 3-(,5-dimethyl-2-hydroxybenyl)-6,8-dimethyl-3,4-dihydro-(2H)-1,3,-benzoxazine and either 2,4-xylenol or 2,6-xylenol was studied by C-13 n.m.r. techniques, to model the reactions between benzoxazine intermediates and free ortho-and para-phenolic sites in the curing of novolac resins with hexamethylenetetramine (HMTA). The results indicate that 2,4-xylenol can directly react with benzoxazine intermediate at low temperature via several pathways to form methylene linkages between phenolic rings. 2,2'-Methylene-4,4',6,6'-tetramethyldiphenol is the dominant product after heating benzoxazine and 2,4-xylenol to 205 degrees C. However, no reaction occurs between the benzoxazine and 2,6-xylenol before the decomposition of the benzoxazine structure. At higher temperatures, 2,6-xylenol reacts with the decomposition species of the benzoxazine to form para-para and ortho-para methylene linkages between phenolic rings, together with ortho-ortho methylene linkages generated from the decomposition of the benzoxazine. Minor amounts of nitrogen-containing structures, such as amines, amides and imines, were also formed. The results when applied to novolac/HMTA systems provide possible reaction mechanisms and pathways from the benzoxazine intermediates and vacant ortho-and para-phenolic reactive sites leading to the final cross-linking network. (C) 1997 Elsevier Science Ltd.