The properties of the birefringent interphase in the laminates of unsaturated polyester resin were characterized using polarized microscopy and a birefringence compensation technique. The thickness of the interphase was found to depend on the casting conditions, and values ranging from 0.004 to 0.25 mm were observed. The location and dependence of the thickness on the curing conditions suggested that the interphase formed due to the diffusion of constituents from the liquid resin of the second layer into the previously cured layer. The index of refraction of the interphase was higher than that of the polyester above and below it. There was a sharp front marking a boundary of the interphase from the rest of the resin. These observations indicated characteristics of non-Fickian interfacial diffusion. Qualitative estimation of the birefringence in the interphase by interference colors indicated the first-order optical retardation. The birefringence was hypothesized to be due to residual stresses resulting from the fabrication procedure. The birefringence was quantified using a Berek compensator to measure the stresses. The approximate stress values were found to be in the range of 10-40 MPa.