A study was made of miscible polymer blends of deuterated polystyrene (d-PS) and tetramethylbisphenol-A polycarbonate (TMPC). The Flory interaction parameter chi was obtained from the relation between mutual and tracer diffusion coefficients, (D) over tilde and D*, which were measured by forward recoil spectrometry. The temperature dependence of diffusion at PS weight fractions w of 0.25 and 0.5, and the composition dependence at temperatures 45 degrees C above the glass transition temperature, T-g, were investigated. A stronger dependence of chi on both temperature (at w = 0.5) and composition was observed in comparison with other miscible binary polymer blends involving PS. Analysis using the generalized lattice-fluid model of Sanchez and Balazs(1) showed that the incorporation of a significant specific interaction is needed to explain the temperature dependence of chi. The diffusion coefficients obtained in the one-phase region were extrapolated to the two-phase region, and these were compared with the effective diffusion coefficient extracted from phase separation dynamics measured by light scattering.(2) A significant discrepancy between the extrapolated and effective diffusion coefficients was observed.