The objective of this study was to examine the effects of interfacial chemistry on the interfacial micromechanics of cellulose fiber/polymer composites. Different interfacial chemistries were created by bonding polystyrene (a common amorphous polymer) to fibers whose surfaces contained different functional groups. The chemical compatibility within the interphase was evaluated by matching the solubility parameters (3) between the polymer and the induced functional groups. The physico-chemical interactions within the interphase were determined using the Lifshitz-van der Waals work of adhesion (W-a(LW)) and the acid-base interaction parameter (Ia-b) based on inverse gas chromatography (IGC). The micromechanical properties of the fiber/polymer interphase were evaluated using a novel micro-Raman tensile test. The results show that the maximum interfacial shear stress, a manifestation of practical adhesion, can be increased by increasing the acid-base interaction (Ia-b) or by reducing the chemical incompatibility (Delta delta) between the fibers and polymer. A modified diffusion model was employed to predict, with considerable success, the contribution of interfacial chemistry to the practical adhesion of cellulose-based fibers and amorphous polymers. The increased predictability, coupled with the existing knowledge of the bulk properties of both fibers and matrix polymer, should ultimately lead to a better engineering of composite properties.