During thermosetting polymer composites processing, the presence of the reinforcing fibers significantly alters the cure characteristics via several microscale processes, forming a fiber/matrix interphase region with different chemical and physical properties from the bulk resin. The interphase composition is an important parameter that determines the microstructure and properties of the composite. The mechanisms contributing to the interphase development during processing include mass transport processes of adsorption, desorption, and diffusion near the fiber surface, which are accompanied by simultaneous cure reaction between the resin components. A model is developed in this paper to describe the coupled mass transfer and reaction processes leading to interphase formation, and to predict the evolution of the interphase concentration with time.