AbstractKinetic evidence in thermomechanical analysis experiments and carbon‐13 nuclear magnetic resonance spectroscopy (13C NMR) evidence indicates that the strength of a joint bonded with UF (urea–formaldehyde)/polymeric 4,4'‐diphenylmethane diisocyanate (pMDI) glue mixes is improved by coreaction of the methylol groups of UF resins with pMDI to form a certain number of methylene cross‐links. The formation of these methylene cross‐links is predominant, rather than formation of urethane bridges which still appear to form but which are in great minority. This reaction occurs in presence of water and under the predominantly acid hardening conditions, which is characteristic of aminoplastic resins (thus, in presence of a hardener). Coreaction occurs to a much lesser extent under alkaline conditions (hence, without UF resins hardeners). The predominant reaction is then different in UF/pMDI adhesive systems than that observed in phenol‐formaldehyde (PF)/pMDI adhesive systems. The same reaction observed for UF/pMDI system at higher temperatures has also been observed in PF/pMDI systems, but only at lower temperatures. The water introduced in the UF/pMDI mix by addition of the UF resin solution has been shown not to react with pMDI to an extent such as to contribute much, if at all, to the increase in strength of the hardened adhesive. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 3681–3688, 2002