The water and solvent sorption behavior of epoxy/rubber polymer blends of various structures are discussed in this chapter. The sorption phenomena are explained in these blends with respect to the surrounding temperature, relative humidity, and moisture concentration profiles based on parameters like diffusion, part and laminate thickness, weight gain due to absorption over time, and saturation equilibration. A reasonable correlation is attempted through Fickian and non-Fickian absorption models for epoxies, rubbers, their blends, and the observed properties. A brief note is presented on the viscoelastic models of environmentally affected blends. The observed changes in the physical, chemical, mechanical, and electrical properties of the blends are documented. Here again, distinction is made between the sorption characteristics of solid rubber/epoxy and liquid rubber/epoxy blends. The hygrothermal performance of fiber composites fabricated with epoxy/rubber blends as the matrix materials is also discussed and presented along with their applications. Limitations, implications, and suggestions for future work have been included at the end of the chapter.