In this paper we examine the interphase in a system which consists of two polymers that are compatible in the melt but undergo phase separation due to the crystallization of one polymer in lamellar semicrystalline morphology. The problem is formulated on a cubic lattice in the Bragg-Williams mean-field approximation, under the assumption that the asymptotic amorphous phase of the system corresponds to the most favorable (for mixing) 50/50 composition. We find that the dissipation of the crystalline order through the interphase of partial order is little affected by the interaction energies between the two polymers. On the other hand, the interfacial region of varying polymer composition is strongly affected by the interaction parameter Χab, with the interfacial thickness varying with the reciprocal of ixabi1/2. Hence, binary polymer mixtures with xab 0, = which are miscible in the melts, are predicted to undergo nearly complete phase separation upon crystallization of one component, in agreement with experiments. © 1989, American Chemical Society. All rights reserved.