Current research on wood reinforcement has focused on the use of fiber-reinforced plastic (FRP) strips or fabrics bonded to wood members. Although significant increases in stiffness and strength have been achieved by this reinforcing technique, there is a concern about the reliable performance of the wood-FRP interface bond, which can be susceptible to delamination. The objective of this study is to present a combined analytical/experimental study to evaluate the effect of moisture on fracture toughness of composite/wood bonded interfaces under Mode I loading. A contoured double cantilever beam (CDCB) specimen is used to characterize the fracture toughness of both wood-wood and wood-FRP samples. The specimens are designed by the Rayleigh Ritz method to achieve a linear rate of compliance with respect to crack length and are calibrated experimentally and also analytically by the finite element method. Both wood-wood and wood-FRP samples are tested under dry and wet conditions, and bonded interface fracture toughness data under Mode I loading are obtained. The guidelines and procedures for the modeling and design of CDCB specimens for hybrid or dissimilar adherends using a Rayleigh-Ritz model are presented briefly, and a modified Rayleigh-Ritz method is further developed. The effect of moisture on fracture toughness is evaluated, and increases in interface fracture toughness are observed due to moisture absorption for wet wood-wood and wood-FRP samples; the toughening of the interface under moisture is due mainly to a much more plastic fracture failure mode of the interface.