An extension of the mean-field approach for the case of two statistical copolymers is described which allows the analysis of copolymer-copolymer blends of the type A(x)B(1-x) + CyB1-y where the copolymer (A(x)B(1-x)) with the mole fraction of monomer A (x(A))= constant is miscible with the (CyB1-y) copolymer between y = 1 and a single composition limit y = y(1), where y(1) > 0. Mixtures of poly(alpha-methyl styrene-co-acrylonitrile) copolymer of azeotrope composition (28.8 wt% acrylonitrile) mixed with statistical methyl methacrylate/acrylonitrile copolymers have been studied and one-phase blends have been found with methyl methacrylate/acrylonitrile copolymers of composition up to 21 wt% acrylonitrile. Estimated segmental interaction parameters acting in the system were positive. (C) 2000 Elsevier Science Ltd. All rights reserved.