The perturbed hard-sphere-chain (PHSC) equation of state for copolymer systems is applied to binary homopolymer blends and to mixtures of homopolymer and copolymer. Theoretical results for the effect of copolymer composition on lower critical solution temperature (LCST) are compared with experiment for homopolymer/copolymer mixtures containing poly(methyyl methacrylate-co-styrene) (MMAco-S) and poly(acrylonitrile-co-styrene) (AN-co-S) random copolymers. The intersegmental parameters for the MMA-S and AN-S pairs are obtained from the miscibility maps containing MMA-co-S and AN-co-S random copolymers, respectively, differing in copolymer compositions. The homopolymer/copolymer systems studied in this work exhibit LCSTs in the experimentally accessible temperature range as the copolymer approaches 100% polystyrene. The PHSC equation of state predicts that the LCST of a homopolymer/copolymer system exhibits a maximum as the copolymer composition varies. Unique values are assigned to the intersegmental parameters for a given pair of segments.