The relative surface coverage of components of a series of symmetric polystyrene/poly(methyl methacrylate) (PS/PMMA) diblock copolymers solution cast from toluene was studied by X-ray photoelectron spectroscopy. The copolymers, which ranged in total molecular weight from 27 000 to 535 000, are microphase separated into domains of pure PS and pure PMMA. The surface energy of PS is a fraction of a dyne per centimeter less than that of PMMA. The surface composition of PS, θPS, was in excess of its bulk value of 50% and increased as the total molecular weight of the copolymer, N, increased. This dependence could be described by an equation of the form ϕPS = a-bN-1/2where a and b are constants. © 1989, American Chemical Society. All rights reserved.