Practical considerations emphasize the desirability of formulating an adequate equation of state for polystyrene. A fairly satisfactory one is this simplified version of van der Waals' equation,(P+&pgr;)(V−&ohgr;)=nRT,where &pgr; is the internal pressure (or cohesive energy density) and &ohgr; is the volume at absolute zero, by extrapolation. Evaluating constants from thermal expansion data and expressingPin p.s.i.,Vin cc per gram, andTin degrees Kelvin, this becomes(P+27,000)(V−0.822)=11.6T.Agreement with compressibility measurements, which are described in some detail, is good. Discrepancies with room temperature values of sound velocity and cohesive energy density by swelling measurements are discussed briefly.