Elastic recoil detection was used to study the segregation of low molecular weight symmetric diblock copolymers of polystyrene (PS) and poly(methyl methacrylate) (PMMA) to the interface of PS and PMMA homopolymers. Copolymer chains composed of 292 and of 262 segments were used. The PS and PMMA homopolymers were composed of 18 000 and 13 000 segments, respectively. At the concentrations studied, Σi−1, the excess number of copolymer chains per unit area at the interface of the homopolymers, varied linearly with, ϕc the volume fraction of copolymer chains in the bulk. The solubility of the copolymer chains was found to be slightly higher in the PMMA phase than in the PS phase. The results are compared with predictions based on modifications of mean field arguments of Leibler. The predicted dependence for the situation in which the density of copolymer chains at the interface is low is in good agreement with the experimental data. The predictions are based on assumptions that both the excluded volume interactions that lead to swelling of the copolymers at the interface and the elastic entropy contributions that limit the swelling of the copolymers are not important. An estimate of the Flory interaction parameter that was made based on the data and the theory is in good agreement with the value determined from small-angle neutron scattering of the PS/PMMA system. Finally, we estimated the reduction in interfacial tension that results when the copolymer chains segregate to the interface. It is consistent with surface tension data on similar systems. © 1991, American Chemical Society. All rights reserved.