The viscosities of polystyrene melts containing three different dissolved gases, carbon dioxide, and the refrigerants R134a (1,1,1,2-tetrafluoroethane) and R152a (1,1-difluoroethane) are investigated at pressures up to 20 MPa. These pressures reach near-critical and supercritical conditions for the three gas components, and produce polymer-gas solutions containing up to 10 wt% gas. The measurements are performed in a sealed high-pressure capillary rheometer at 150 and 175 degrees C, and at shear rates ranging from 1-2,000 s(-1). Very large reductions in melt viscosity are observed at high gas loading; at 150 degrees C, 10 wt% R152a reduces the Newtonian viscosity by nearly three orders of magnitude relative to pure polystyrene. The viscosity data for all three polystyrene-gas systems follows ideal viscoelastic scaling, whereby the set of viscosity curves for a polymer-gas system can be scaled to a master curve of reduced viscosity vs. reduced shear rate identical to the viscosity curve for the pure polymer. The Viscoelastic scaling factors representing the effect of dissolved gas content on rheological behavior are found to follow roughly the same variation with composition for all three polystyrene gas systems.