The pressure decay method has been used to study the diffusivity and solubility of carbon dioxide gas in polystyrene at high temperature and pressure. Preliminary data have been generated at pressures up to 1000 psi, at a temperature of 200⁰C. Literature values (Sato, 1996, 2001) have been shown to be in good agreement with the results from this study. Several sources of error, and possible solutions, have been identified, which would further improve the repeatability of the results.