The effect of CO2 solubility on T-g versus CO2 partial pressure behavior is presented for poly(methyl methacrylate) (PMMA), polystyrene (PS), and a random copolymer of the two with 60% by weight methylmethacrylate. Two new features of T-g versus pressure behavior have been discovered experimentally, confirming previous theoretical predictions (Condo, et al. Macromolecules 1992, 25, 6119). The results for the copolymer, poly(methyl methacrylate-co-styrene) (SMMA60), reveal a novel "z-shape" in the T-g behavior as a function of pressure, including a narrow pressure region in which three glass transitions take place upon changing the temperature isobarically (type III behavior). In contrast, only one glass transition temperature for each pressure is present for PS, without an extremum (type II behavior). The results are modeled accurately with lattice fluid theory and explained in terms of the combined effects of pressure and temperature on the solubility of the diluent in the polymer.