Thermodynamic and kinetic treatments are employed to analyze the role of preferential adsorption and diffusion in the formation of interphase regions in thermosetting composities. The thermodynamic analyses show that a driving force exists for the preferential adsorption of amines onto carbon fiber surfaces in amine-epoxy systems. Scaling analyses, based on kinetic considerations, lead to the identification of reaction-diffusion time regimes that control interphase formation in sized and unsized fiber systems. These considerations provide guidelines for the selection of material formulations and processing conditions that may be used to tailor behavior of the interphase region in thermosetting composites.