The solubility and diffusivity of ethylene in semicrystalline polyethylene were experimentally measured using a magnetic suspension microbalance. The sorption measurements were carried out at temperatures up to 80degreesC and pressures up to 66 atm. The experimentally measured solubilities were found to decrease with increasing temperature and increased with ethylene pressure in good agreement with the predictions of the Sanchez-Lacombe lattice-fluid model. The diffusivity of ethylene in sernicrystalline polyethylene films was estimated from the reduced sorption curves using the half-time method. The experimentally determined diffusivities were compared with theoretical values predicted by a new molecular hybrid model, which combines the characteristic features of the Pace-Datyner diffusion model with those of the Kulkarni-Stern free-volume model. The ethylene diffusion coefficient was found to increase with temperature and/or the ethylene-sorbed concentration.