Simple theoretical models show that concentration of a compatibilizing A-B block copolymer at the interface between the phases of A and B homopolymers in immiscible polymer blends increases with increasing chain length of copolymers or homopolymers. Copolymers with block lengths comparable to chain length of the blend components reduce interfacial tension most efficiently. Distribution of the block copolymer between the interface and the bulk phases is studied assuming that it is controlled by the rules of equilibrium thermodynamics. Analysis is based on Leibler's mean field theory and its modification by Noolandi. The analysis shows that the distribution of the copolymer between the interface and the bulk phases changes in a broad range depending on the system parameters. Substantial part of the added copolymer can be localized at the interface in contrast to assumptions used in previous analyses. (C) 2018 Elsevier Ltd. All rights reserved.