Mixing thermodynamics in miscible blends of polystyrene CPS) and tetramethylbisphenol-A polycarbonate (TEvIPC) was investigated using liquid state pressure-specific volume-temperature (P-V-T) properties of both purl components and mixtures. The equation-of-state theories used were (1) the lattice fluid model of Sanchez and Lacombe, (2) the model of Flory, Orwoll, and Vrij, and (3) the modified cell model suggested by Del and Walsh. The composition dependence of characteristic pressure was first used to extract the interaction parameter (Delta P*) and Flory interaction parameter expressed in the second derivative of the free energy of mixing (chi(sc)) It Was found that the sign of chi(sc) was negative and the magnitude of it was always significantly larger than the values obtained by small-angle neutron scattering (Yang H, O＇Reilly JM. Mater Res Soc Symp Proc 1987;79:129) and diffusion measurements (Kim E, Kramer EJ, Osby JO, Walsh DJ. J Polym Sci, Part B: Polym Phys 1995;33:467), indicating that the blend P-V-T properties grossly overestimate the attractive interaction. On the other hand, the chi(sc) predicted from the characteristic temperature was also large but had a positive sign. These results were similar to what had been found in PS/PVME blends by Ougizawa and coworkers (Ougizawa T, Dee GT, Walsh DJ. Macromolecules 1991;24:3834). While the thermal expansion coefficient began to increase as temperature is raised above the lower critical solution temperature (LCST), the volume contraction upon mixing was observed above as well as below the LCST. This observation implies that two dissimilar chains are packed together to form a certain stable stereo structure. We also note that the decreased change in core volume rather than the presence of large Delta P* causes the volume contraction upon mixing. (C) 2000 Elsevier Science Ltd. All rights reserved.