The effect of uniaxial orientation on the free-volume and oxygen-transport properties of a propylene copolymer with 4.5 wt % ethylene was examined. The free-volume hole size and hole density were measured with positron annihilation lifetime spectroscopy. Subsequently, the free-volume characteristics were correlated with the oxygen-transport properties. Orientation had only a small effect on the total amount of free volume: a small increase in the hole density was offset by a small decrease in the hole size. As a result, the oxygen solubility and amorphous-phase density were unchanged by orientation. However, a pronounced decrease in the oxygen diffusivity when the draw ratio exceeded 6 indicated a change in the dynamic free volume. This was attributed to an increasing number of taut tie chains, which retarded oxygen diffusion. The reduced amorphous chain mobility was also manifest in the increased glass-transition temperature, decreased bulk thermal expansivity, and decreased expansivity of free-volume holes. (c) 2005 Wiley Periodicals, Inc.