Heterogeneous catalytic hydrogenation can improve the chemical, thermal, and oxidative resistance of polystyrene.  The hydrogenation reaction requires the polystyrene molecules to diffuse into microscopic catalyst pores. When diffusing into the catalyst, the polymer coils experience mass transfer limitations associated with the high viscosity of the polystyrene solution.  Supercritical fluids have been shown to increase mass transfer rates by enhancing diffusivity and lowering viscosity.  Supercritical CO₂ was investigated as a co-solvent with 76/24 wt% trans/cis -decahydronaphthalene (decalin) to determine the effect CO₂ would have on the polystyrene solution viscosity.  Phase behavior and viscosity measurements were obtained for decalin, supercritical CO₂ expanded decalin, polystyrene in decalin, and polystyrene in supercritical CO₂ expanded decalin.  The effects of the decalin isomers on viscosity and phase behavior were investigated to obtain binary interaction parameters and pure component parameters for the solvent system.  The viscosity data of the solutions compares well to the Free Volume Model (FVM) for viscosity.  Both the model and the experimental data show a significant decrease in viscosity for the polystyrene in supercritical CO₂ expanded decahydronaphthalene.  Future work will investigate the effect of hydrogen on the solution properties and determine the kinetic properties of the hydrogenation reaction.