The gas sorption and transport properties of poly(octadecyl acrylate) are described for the semi-crystalline and molten states. The change in permeability upon traversing the melting point, or permeation switch, was observed to be as high as two orders of magnitude depending on the penetrant. Analysis of gas transport is explained using the Michaels and Bixler model for semi-crystalline polyethylene. This analysis shows chain immobilization of the amorphous phase by surrounding crystals to play a significant role in describing the penetrant-dependent permeation switch in this side-chain crystalline polymer. Traditional two-phase models describing crystallites as impenetrable barriers with no effect on amorphous phase properties may explain solubility in this system but do not describe diffusion. The dependence of transport parameters on polymer composition, e.g. side-chain length, is also discussed. (C) 2000 Elsevier Science Ltd. All rights reserved.