The segment density distribution of symmetric, diblock copolymers of poly(styrene), PS, and poly(methyl methacrylate), PMMA, denoted P(S-b-MMA), were investigated by neutron reflectivity. Selective labeling of either block of the P(S-b-MMA) or of the PS or PMMA with deuterium provided the contrast necessary to isolate the distribution of the segments of the components at the interface. Results from a series of experiments were used to solve a simultaneous set of linear equations that yielded the segment density profiles of the PS and PMMA segments of the homopolymers and copolymers at the interface. It was found that the interface formed between the PS and PMMA segments could be described by a hyperbolic tangent function with an effective width of 75 Å. This width is 50 % broader than that found between the PS and PMMA homopolymers in the absence of the diblock copolymer and between the PS and PMMA lamellar microdomains of the pure P(S-b-MMA) in the bulk. A significant penetration of the PS and PMMA homopolymers into the interfacial region was also found. The area occupied by the copolymer at the interface between the homopolymers is 30% larger than that of the copolymers in the bulk, lamellar microstructure. © 1991, American Chemical Society. All rights reserved.