The coabsorptions of ethylene and iso-pentane, and ethylene and n-hexane were measured by the use of a pressure decay method. The solubility data of ethylene-iso-pentane and ethylene-n-hexane in semicrystalline polyethylene (PE) of crystallinity of 48.6% were obtained at temperatures 70, 80, and 90 degrees C, and the total pressure 2 MPa, iso-pentane partial pressure 80-190 KPa, n-hexane partial pressure 20-90 KPa. The presence of iso-pentane or n-hexane in the corresponding ternary system leads to increase the solubility of ethylene, while the solubility of iso-pentane or n-hexane remains unchanged with an increase of the ethylene partial pressure, even slightly decreases. Assumed that the presence of iso-pentane or n-hexane decreases the crystallinity of the polymer sample, a coabsorption model was built to model the solubility of each gas in the ternary systems. The relative root mean square errors of the coabsorption model for ethylene-iso-pentane-PE system and ethylene-n-hexane-PE system are 5.13% and 4.64%, respectively. (c) 2007 Wiley Periodicals, Inc.