Nuclear reaction analysis (NRA) was used to study the segregation of polyisoprene (PI)-deuterated polystyrene (dPS) diblock copolymers to the interfaces formed by high molecular weight (M) polystyrene (PS, M = 3 x 10(6)) with vacuum, silicon, and gold. Two types of diblock copolymers, a symmetric PI(M=10(4))-dPS(M=10(4)) diblock and a strongly asymmetric PI(M=10(4))-dPS(M=10(6)) diblock, were investigated, and their segregation isotherms were determined as a function of their bulk concentration within the PS homopolymer. Isotherms were analyzed in terms of a mean-field approach due to de Gennes and Leibler and the numerical results of a self-consistent mean-field model. Both approaches were found to provide a consistent description of the data.