Segregation to interfaces in two-phase blends containing diblock copolymers is fundamental to understanding the properties of these interfaces. In this study, the effect of homopolymer molecular weight on the segregation is examined by forward recoil spectrometry (FRES). In the case of Pa > Na. where Pa and Na are the polymerization indices of the homopolymer A and the A block of the copolymer, respectively, it is found that the interfacial areal density of block copolymer chains (vi) saturation is not affected significantly by Pa. This situation corresponds to the case where the “brush” of A block chains is not penetrated strongly by the A homopolymer, the so-called dry brush case. However, a lower vi is observed in the case of Pa < Na, the so-called wet brush case. While Leibler's brush theory1 can reproduce the shape of the segregation isotherm if the Flory interaction parameter ϰ is allowed to vary with Pa, a self-consistent meanfield theory2 can predict the observed isotherm using a single value of ϰ. The critical micelle concentration, ϕcmc is obtained from the onset of surface segregation and is found to decrease as the molecular weight of the homopolymer matrix increases. © 1992, American Chemical Society. All rights reserved.