The phase behavior of blends of SMMA copolymers with other SMMA copolymers of different compositions, with monodisperse PS homopolymers, and with monodisperse PMMA homopolymers was studied using static light scattering to assess the state of miscibility at three different temperatures: 120, 150, and 180 degreesC. Contrary to prior reports, the phase diagrams were found to be consistent with the binary interaction model. Quantitative evaluation of the binary interaction energy density between S and MMA, B-S/MMA, was made at each temperature by analyzing the miscibility data using the copolymer/critical molecular weight and copolymer composition mapping methods. Values of BS/MMA determined by two different approaches agree well. The slight temperature dependence of BS/MMA is well described by the Sanchez-Lacombe lattice fluid theory using a constant bare interaction energy DeltaP*(S/MMA) by considering the temperature dependence of the characteristic parameters for PS and PMMA. Quantum mechanical calculations of the total partial charges for different triads clearly show that the electrostatic charge distribution within S and NMA repeat units remains almost unchanged as their neighboring segments change chemical identity. The experimental and theoretical results are inconsistent with the so-called 'screening', 'asymmetric miscibility' or sequence distribution effects. (C) 2003 Elsevier Science Ltd. All rights reserved.