Polymeric diphenylmethane diisocyanate (pMDI) has been used for making wood board composites for over 30 years, although its growth in application has been limited mainly by the perception that it is a high-cost alternative to formaldehyde-based adhesives. Increased use of pMDI adhesive in making wood composites will require optimization of the process variables involved. One such factor is the unresolved question of the interfacial aspects most responsible for building strong wood composites. Some argue that adhesive surface coverage is primarily responsible for board strength; others assert that penetration of the pMDI into the wood matrix is a primary characteristic of strong boards. This article wilt provide evidence that the interfacial aspect most important in building board strength is surface coverage of the wood particles or fibers. Experiments controlling the depth of pMDI penetration into wood matrices and then measuring their adherend strength were performed. We also calculated the potential efficiencies to be obtained from maximizing the surface coverage and minimizing the matrix penetration using a spherical sector model. Neither high nor low pMDI viscosity offered a significant advantage in surface covering pine or oak, although it is speculated that minimizing the droplet size of pMDI in the resination step could lead to substantial efficiency improvements in adhesive coverage. (c) 2005 Wiley Periodicals, Inc.