Thin film mixtures of high and low molecular weight symmetric poly(styrene-b-methyl methacrylate) diblock copolymers were investigated by neutron reflectivity. The preferential interaction of the block components with the surface and substrate causes the mixtures to organize upon annealing into alternating lamellar microdomains of PS and PMMA oriented parallel to the substrate surface. By deuterating the PMMA blocks of both mixture components, the periodicity and interfacial width were determined as a function of the mixture composition. The lamellar period, d, of the mixture was found to scale approximately with the number-average molecular weight as d similar to M(n)(2/3) Via selective deuteration, the copolymer block distributions. within the domains were obtained by combining results from complementary experiments. In all cases investigated, nonuniform profiles were observed whereby the shorter copolymer chains localized to the PS/PMMA interface regions while longer chains enriched the domain centers.