n-Hexane desorption measurements were pet-formed on heterogeneous poly(ethylene-co-octene)s with hexyl branch contents between 0.8 and 3.9 mol% and crystallinities between 30 and 60%, Crystal core contents obtained by Raman spectroscopy were lower than the density-based crystallinity, particularly For the samples with the highest degree of branching. A modified Cohen-Tumbull-Fujita free volume model adequately described the diffusivity data. The free volume of the penetrable phases was strongly dependent on their total volume fraction, suggesting the presence of an interfacial penetrable component with low fractional free volume. The dependence of the fractional free volume of the penetrable phases on the phase composition suggests that mass transport takes place from the liquid-like component to the interfacial component and that the penetrant molecules are trapped at the interfacial sites. The decrease in geometrical impedance factor with increasing crystallinity may be explained by the presence of extraordinarily wide dominant crystal lamellae in the heterogeneous low crystallinity samples. The saturation concentration of n-hexane in a wide range of polyethylenes (including the heterogeneous poly(ethylene-co-octene)s, linear polyethylenes and poly(ethylene-co-butene)s reported earlier; crystallinity range: 30-94%) showed a complex non-linear dependence on crystallinity that was qualitative in accordance with rubber elasticity theory considering also molecular cilia. (C) 2001 Elsevier Science Ltd. All rights reserved.