We examined the influence of film thickness and composition on the effective T-g of compatible thin film mixtures of polystyrene (PS) and tetramethylbisphenol-A polycarbonate (TMPC) on SiOx/Si substrates using spectroscopic ellipsometry. Our measurements reveal that while the T-g of TMPC films increased with decreasing film thickness, h, the effective T-g of thin film mixtures of PS and TMPC decreased with decreasing film thickness. In these mixtures, T-g was independent of film thickness at large h. We also found that while the T-g of bulk mixtures of TMPC/PS exhibited large negative deviations from additivity with composition, such deviations were negligible in the thin film mixtures. The thickness dependence of T-g is compared with theory.