Previous theories describing the phase behavior of copolymer blends have ignored the sequence distribution of monomer unit.a in the copolymer. We introduce a parameter, 8, that describes the binary sequence distribution of the monomers in a copolymer chain. By varying 8, we can describe a block, random, or alternating copolymer. It is assumed that the interaction energy between a monomer of homopolymer C and the monomer A (or B) in the copolymer AB is mediated by the nearest neighbors chemically bonded to the A (or B) structural 

unit. It is found that the sequence distribution may significantly affect the degree of compatibility between the polymers AB and C. For a fixed composition, there is an optimal range of(} values (or sequence distributions) 

for which the C/AB system is miscible.