A methodology for predicting the formation and influence of interphase regions in composite materials is illustrated through an investigation of the relationship of sizing-induced interphase regions to the development of residual thermal stresses in a carbon fiber epoxy-amine composite. Fiber surface and sizing induced concentration gradients in the epoxy-amine system were predicted. Material property data was measured for bulk epoxy-amine systems corresponding to the predicted interphase concentrations and the properties mapped into property profiles in the vicinity of the fiber surface. Micromechanical models were used to predict residual thermal stresses for carbon fiber epoxy-amine composites with these interphase properties. The analyses predict that the thermal stress state is significantly affected by modulus variations in the interphase region. The variations in the properties of the interphase material can be affected through processing conditions and/or material selections.