A hybrid model, which combines the characteristic features of the Pace-Datyner molecular model with those of the Kulkarni-Stern free-volume model, was developed to assess the effect of temperature, penetrant concentration, and polymer crystallinity on penetrant diffusivity. The predictive capabilities of the proposed model were tested by a direct comparison with experimental data. The diffusivity of ethylene and propylene vapors in semicrystalline polyethylene and isotactic polypropylene was experimentally measured using a magnetic suspension microbalance. Sorption kinetic measurements were carried out at temperatures up to 80 degrees C and pressures up to 80 atm. The diffusivity was found to increase with temperature and penetrant concentration. Apparently, there was a very good agreement between the theoretical values predicted by the new hybrid diffusion model and the experimentally determined diffusivities. (c) 2007 Wiley Periodicals, Inc.