The equilibrium segregation of deuterated polystyrene–poly(2-vinylpyridine) diblock copolymers to interfaces between high molecular weight polystyrene and poly(2-vinylpyridine) homopolymers was measured by forward recoil spectrometry. The dependence of the integrated segregation on the equilibrium copolymer concentration in the PS phase is compared to predictions from a mean-field theory in which the copolymer chemical potential is the relevant parameter. Predictions from the theory are quantitatively accurate for values of the copolymer chemical potential, which are below a certain limiting value associated with the formation of block copolymer micelles. The segregation behavior in the regime where micelles are present is complicated by a strong tendency for micelles to segregate to the free polystyrene surface and by a weaker tendency for micelles to segregate to the interfacial region. Values of the copolymer chemical potential at the micelle transition are obtained from a careful analysis of the data and are in reasonable agreement with predictions from a simplified theory of micelle formation. © 1990, American Chemical Society. All rights reserved.