Sorption and dilation properties of polymer-gas systems involving poly( ethylene-co-vinyl acetate) and N-2, CH4, or CO2, have been investigated at pressures up to 50 atm at temperatures of 10-40 degrees C. Sorption isotherms for low-solubility gases (i.e., CH4 and Nz) can be described by Henry’s law, and those for high-solubility gas (i.e., CO2) by Flory-Huggins dissolution equation. Dilation isotherms are similar in contour to the corresponding sorption isotherms. From the obtained sorption and dilation data, partial molar volumes of the gases in the polymer were determined as a function of temperature. Thermal expansivity of dissolved CO2 molecules was estimated at ca. 2.4X10(-3)degrees C-1 from the temperature dependence of partial molar volume. The expansivity is smaller than that of liquid CO2 and larger than those of the polymer and organic liquids.