Summary Kinetic evidence from thermomechanical analysis experiments and 13C NMR evidence were used to show that the strength of a joint bonded with UF/pMDI glue-mixtures is improved by coreaction between pMDI and UF resin methylol groups forming methylene cross-links. These are predominant, as opposed to the formation of urethane bridges which still appear to form, but are in great minority. The coreaction occurs in the presence of water and under the predominantly acidic hardening conditions characteristic of aminoplastic resins (in presence of a hardener). Coreaction occurs to a much lesser extent under alkaline conditions (without UF resins hardeners). The predominant reaction is therefore different in UF/pMDI and PF/pMDI adhesive systems. The same reaction observed for the UF/pMDI system at higher temperatures was also observed in PF/pMDI systems, but only at lower temperatures. The water introduced into the UF/pMDI mixture by addition of the UF resin solution was shown not to react with pMDI to increase the strength of the hardened adhesive to a significant degree.