A molecular thermodynamic model for copolymer mixtures has been established by combining a reference term for hetero-nuclear hard-sphere chain fluids with Alder et al．’s square-well attractive potential served as a perturbation term．The segment parameters of polymer (including segment number

segment diameter and interaction energy between segments) can be obtained by fitting pVT data of pure homopolymer．The adjustable interaction parameters between segments of polymers can be determined from liquid-liquid equilibrium data．This model can be used to correlate coexistence curves for homopolymer/copolymer systems．The effect of copolymer composition on lower critical solution temperature (LCST) and phase separation temperatures are investigated and compared with experiment results for r1/(CXD1-X)r2 copolymer mixtures．For (AXB1-X)r1／(AYB1-Y)r2 copolymer mixtures

the interaction parameters between segments of polymers can be obtained from the boundary between miscible and immiscible regions．It was found that the model can satisfactorily be used to correlate phase behaviour including coexistence curves

miscibility windows and miscibility maps for (AXB1-X)r1／(CYB1-Y)r2 copolymer mixtures

only a pair of adjustable interaction parameters need to be determined from the boundary between miscible and immiscible regions.