The Dow Chemical Company has recently introduced a new family of copolymers of ethylene and styrene identified as ethylene-styrene interpolymers (ESIs). The ability of the group-contribution lattice-fluid equation of state (GCLF EoS) to predict the volumetric properties of a series of ESIs of different compositions, the solubility parameter of a given copolymer, and the solubility of toluene was tested. The two latter experiments were conducted in the temperature range 50-150 degreesC. On average, the error in density was less than 1.1% in the temperature and pressure ranges of 100-250 degreesC and 6.9-690 bar, respectively. The solubility parameter of the polymer was measured using inverse gas chromatography. The solubility parameter was found to have a strong temperature dependency that the model could not reproduce accurately. At 25 degreesC, however, the model predictions were found to be in the range of the literature data. The GOLF EoS gave good predictions for the solubilities of the vapor's at low styrene contents. Discrepancies at high styrene contents were attributed to a less accurate description of the sorption of solvents in pure polystyrene. The investigation shows that crystallinity has an influence on the solvent solubilities even if they are treated on a crystal-free basis.