The free volume theory of Gerhardt et al.(1) (J. Polym. Sci. B: Polym. Phys. 1998, 36, 1911) is used to predict viscoelastic scaling factors describing the effect of dissolved gas content on the viscosity curves of polystyrene melts swollen with dissolved carbon dioxide and dissolved 1,1-difluoroethane. The predictions of the theory are compared to viscoelastic scaling factors measured by Kwag et al.(2) (J. Polym. Sci. B: Polym. Phys. 1999, 37, 2771) for each system at 150 and 175 degreesC, at concentrations up to 10 wt % of dissolved gas, and pressures ranging up to 22 MPa. The agreement between the theory and experiments is very good for the polystyrene-CO2 system but only fair for the polystyrene-1,1-difluoroethane system. The experimental viscoelastic scaling factor values are also interpreted with the WLF equation to estimate the change in the underlying glass transition temperatures of the polystyrene-gas mixtures. The glass transition temperatures estimated from these rheological data are in very good agreement with values directly measured for polystyrene-CO2 mixtures and with the theory of Condo et al.(3) (Macromolecules 1992, 25, 6119).