The internal reinforcement of Douglas-fir by glass or basalt fibers impregnated with resorcinolphenol-formaldehyde or polymeric diphenylmethanediisocyanate bonding resins has been investigated. The objective was to evaluate the improvement in composite properties by the use of silane coupling agents. Fibers were treated with 3-(4-methoxyphenyl) propyl methyldichlorosilane for use with the phenolic resin, and with 3-aminopropyl triethoxysilane for use with the isocyanate resin. Resin impregnated fiber plies were sandwiched between wood pieces to prepare test specimens with 17 vol. % fiber for measurement of block shear strength, and modulus of rupture (MOR) and modulus of elasticity (MOE) in 3-point bending, and interlaminar shear strength in 5-point bending. The durability under wet conditions was evaluated by subjecting the specimens to an accelerated exposure test (ASTM D2559). It was found that silane application resulted in slight shear strength improvement under dry conditions but much higher increase in shear strength retention after exposure to moisture. Basalt fibers and E-glass fibers gave similar results and comparable properties.