Supercritical fluids (SCF) such as carbon dioxide and nitrogen are of interest as physical blowing agents in the manufacture of plastic foam and as plasticizers to reduce melt viscosity during processing. The processor must know the combined effects of dissolved SCF, pressure, and temperature on the rheological properties of the melt in order to achieve optimum processing conditions. To produce good foam products, polymer blends are sometimes used, for example, linear polypropylene with branched polypropylene. Reliable models for predicting the physical properties of these materials in the presence of a solvent are not available, so experimental data are necessary to evaluate candidate resins. Five polypropylenes were chosen for study: one linear and one branched, plus three blends of the two. For purposes of comparison, data using high-density polyethylene is also presented. The pressure-volume-temperature (PVT) behavior of the samples was determined, to establish their basic phase behavior. To determine the combined effects of blend composition, SCF concentration, pressure, and temperature, a high-pressure sliding-plate rheometer (HPSPR) and two rotational rheometers were used. In the HPSPR the shear deformation, temperature, pressure, and SCF concentration were all uniform. A shear-stress transducer sensed the stress in the center of the sample to avoid edge effects. It was possible to use shift factors for temperature, pressure, and SCF concentration to obtain a master curve. The effect of temperature could be described by the Arrhenius equation, and the effect of pressure was described by the Barus equation. The effect of concentration of CO2 could be modeled using the Fujita-Kishimoto equation. The relative effects of concentration, pressure, and temperature on the viscosity were quantified.