An absorption model of gases in semicrystalline polymer was built that was based on the activity coefficient theory in polymer solution and associated with crystallinity dependent on temperature. The solubility of ethylene, isopentane, and n-hexane in three types of polyethylene (PE) were obtained by the use of a pressure-decay method at temperatures of 333-363 K and pressures of up to 2 MPa, 80-300 KPa, and 19-100 KPa, respectively. Experimental data from three gases in each PE sample were used for the single-parameter fitting, and fitting error was within about 12%. It was found that a single parameter was merely dependent on the properties of PE used. It was shown that, unlike with the Flory-Huggins model and the UNIFAC-M-H method, correlation between the crystallinity of the semicrystalline polymer and temperature had to be taken into account in order for the solubility data of alkane, olefin, and aromatic hydrocarbons in polyethylene to fit well, especially in the temperature range near the melting point of the polymer. The four free-energy contributions to the total gas activity were experimentally determined to be about 47%-60% combined, the free-volume contribution about 12%-25%, and the elastic effect about 22%35%, but the interactional contribution was zero. The contributions changed with the size of the gas molecules. (c) 2006 Wiley Periodicals, Inc.