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Interfacial adhesion across the fiber/matrix interface plays a critical role in controlling the performance of reinforced wood fiber and thermoplastic composites. In this study, the surfaces of cellulosic (rayon, cotton, and wood) fibers were chemically modified with acetic anhydride and styrene-maleic anhydride copolymers, producing different and controlled fiber surface morphologies and chemical functionalities. A specific emphasis was given to the modification to wood fiber surfaces. Dynamic contact angle (DCA) analysis and a microbond test were used to determine the fiber surface free energy and the acid/base interaction and evaluate the interfacial shear strength (ISS) between modified wood fibers and a polystyrene matrix. A hydrophilic/hydrophobic adsorption lattice (HHAL) model was proposed to correlate the fiber surface thermodynamic behavior with the fiber surface modification. Experimental and theoretical analyses indicate that the thermodynamic behavior on the modified wood fiber surface can be described using the HHAL model. The fiber surfaces were altered in terms of morphology, chemical composition, and thermodynamic properties after chemical modification with acetic anhydride and styrene-maleic anhydride copolymers. Combination of DCA and microbond tests can be used to evaluate interfacial adhesion between wood fibers and the polystyrene matrix. For wood fibers treated with acetic anhydride, the improved wetting and spreading of melted polystyrene on the fiber surface is mainly responsible for the improved interfacial adhesion across the wood fiber/polystyrene matrix interface. In contrast, for wood fibers treated with styrene-maleic anhydride copolymers, interfacial adhesion between the fiber and thermoplastic matrix is attributed to the combined effect of the acid/base interaction with the interdiffusion across the wood fiber/polystyrene matrix interface.