Microscopic observations of cross sections of laminates of unsaturated polyesters revealed a birefringent zone (interphase) at the interface. Several observations associated with this interphase were made that predicted either beneficial or detrimental effects of the presence of the interphase in the laminate. A study was conducted to identify the origin and to characterize the properties of the interphase. It was proven that the interfacial diffusion of uncured liquid resin into the previously cured resin formed the interphase. The significant effect of the curing of the second layer on the thickness of the interphase was evident. The diffusion kinetics was dependent on the laminate preparation conditions. A correlation was developed that predicts the properties of the interphase based on crosslinking density of layer I and gel time of the resin of layer II.