The phase behavior of blends of S-nBMA copolymers with monodisperse PS homopolymers was studied at three temperatures: 120, 150 and 180 degreesC. Quantitative evaluation of the binary interaction energy density between styrene (S) and n-butyl methacrylate (nBMA), B-S/nBMA, was made at each temperature by analyzing the miscibility data using the copolymer/critical molecular weight method. The temperature dependence of B-S/nBMA is interpreted by the Sanchez-Lacombe lattice fluid theory using a constant bare interaction energy DeltaP(S/nBMA)*. The BS/nBMA values determined in this study were much larger than those reported by others. Deuteration effects may be the reason for this difference. (C) 2003 Elsevier Science Ltd. All rights reserved.