CP MAS C-13 NMR spectra of hardened resins have shown that urethane bridges derived from the reaction of the isocyanate group with the hydroxymethyl group of urea do form even at fast curing times comparable to what was used in the wood panels industry, in lower proportions than what was shown earlier. Polyureas and biurets obtained from the reaction of isocyanate with water are the predominant crosslinking reactions of pMDI alone and in UF/pMDI resin systems under fast curing conditions. Residual, unreacted isocyanate groups in the hardened network are consistently observed. Their proportion markedly decreases when the original proportion of urea-formaldehyde (UF) resin is high and that of pMDI is low. Under these fast curing conditions, the UF resin appears to self-condense through an unusually high proportion of methylene ether links rather than methylene bridges alone. A marked proportion of residual, unreacted hydroxymethyl groups is also noticeable, initially, in the UF self-condensation network. Direct NMR tests on thin hardboard bonded under fast pressing conditions with different proportions of UF/pMDI confirmed that crosslinking due to polyureas and biurets formation are predominant in the crosslinking of pMDI when alone and in UF/pMDI resin systems. They confirmed that residual, unreacted isocyanate groups are present in the finished panel. Their proportion is higher when the proportion of pMDI in the system is high. The presence or absence of urethanes could not be confirmed directly on the panels as the relevant peaks are masked by the wood carbohydrates signals of wood cellulose and hemicelluloses. (c) 2006 Wiley Periodicals, Inc.