Abstract Samples of a blown high density polyethylene (HDPE) film with high orientation in machine direction were treated at 34, 74 and 114°C for different periods of time. The crystallinity and birefringence of these samples, as well as the sorption curves with toluene vapor, were determined as a function of the treatment temperature and time. The sorption curves showed that the diffusion behaviour of the oriented samples was non-Fickian. The annealing of the samples resulted in a decay of the orientation and a change of the diffusion mechanism. The latter was manifested by the decrease of the exponent n which describes the time dependence of the solvent uptake in the equation: M(t)/M∞ = k t n .