Polymeric diphenylmethane diisocyanate, pMDI, has become an important wood binder in recent years, due to its excellent performance in wood-based composites. However, much still remains unknown about the nature of their bonding mechanism. This research describes efforts to learn more molecular information about the pMDI-wood bondline, and to further improve the bonding performance. In order to correlate molecular phenomena with macroscopic performance of wood-adhesive bondline, low frequency molecular motions in wood were probed using dynamic mechanical analysis (DMA) and ¹³C cross-polarization, magic-angle spinning (CP/MAS) NMR. A correlation between the CP time constant <T_c_h* and the dynamic storage modulus E’ was established for dry wood, but was not valid for wet wood samples. Two types of pMDI with properties similar to commercial resins, except for isomer ratio, was synthesized. The one with isomer ratio similar to commercial resins was analyzed with ¹⁵N CP/MAS NMR. The results show that the pMDI-wood bondline is a heterogeneous complex of urethanes, polyureas, residual isocyanates and biurets. The network structure is controlled by the curing variables such as temperature and time. Urethane formation was detected under relatively mild cure conditions. Thermal decomposition of urethanes (120°C) and polyurets (185°C) were detected. ¹⁵N NMR was demonstrated as a powerful technique, but suffers from signal overlap which prevents a clear evaluation of the relative contributions of urethane and urea formation. Another type of pMDI with higher 2,4’- isomer content was used to investigate the effects of isomer ratio on bonding mechanism. The chemical species found in the heterogeneous bondline are similar, except that urethane formation is less evident here. Relaxation studies show very different behaviors, in which the bondline with higher 2,4’- isomers may have a higher molecular mobility. A fracture toughness test method, contoured double cantilever beams (CDCB), was developed to evaluate the macroscopic performance. They both showed strong bonding, and there was no significant difference found. To explain the strong bonding of pMDI and wood, an interpenetrating polymer network (IPN) theory was hypothesized. ¹³C CP/MAS NMR and DMA were used to evaluate this hypothesis. The results were inconclusive due the limitations of the techniques. / Ph. D.