Dielectric relaxation spectroscopy (DR) was employed to study the dynamics of glassy tetramethylbisphenol A polycarbonate (TMBPA-PC) containing various amounts of tris(2-ethylhexyl) phosphate (TOP) added as diluent molecules. The present data confirm that the γ-relaxation dynamics, at constant frequency, shift to lower temperatures with increasing TOP content, an effect observed for the first time by dynamic mechanical measurements. This behavior of the γ-relaxation in TMBPA-PC plasticized by TOP is to be contrasted with that in bisphenol A polycarbonate (BPA-PC) plasticized by TOP. In the latter case, the γ-relaxation dynamics, at constant frequency, shift to higher temperatures with addition of TOP. These opposite trends of the γ-relaxation dynamics in two similar polycarbonates are surprising. However, the results can be rationalized by the coupling model.