Polystyrene is characterized as a mixture of a finite number of discrete pseudocomponents. Solubilities and partition coefficients of the pseudocomponents in propane and in ethane are measured in semibatch experiments at temperatures ranging from 323 to 453 K and pressures up to 67 MPa. An equation of state based on the statistical associating fluid theory (SAFT) is used to correlate the data. Increasing temperature and pressure are found to increase the solubilities and partition coefficients, but decrease selectivities. The solubility of pseudocomponents is also found to depend on the molecular weight distribution of the polymer.