Small-angle X-ray scattering measurements were performed in order to estimate the interfacial thickness of melt blended poly(styrene)/poly(methyl methacrylate)samples, with and without the corresponding poly(styrene-b-methyl methacrylate) copolymer. The analysis of the scattering profile was based on deviations from Porod’s law using the Koberstein/Morra/Stein (J. Appl. Crystallogr. 1980, 13, 34) graphical method in which the desmearing procedure is avoided. The interfacial thickness was found to increase regularly with the addition of the copolymer, up to 33% when 10 wt % copolymer was added to the homopolymer blend. These results are in agreement with existing theories.