In this paper, the results of using a mass spectrometer technique to measure mixed-gas diffusion through polymer films are presented. Mixtures of oxygen, carbon dioxide and nitrogen are diffused through films of polyethylene with different degrees and type of chain branching. It is shown that in the case of pure gases Henry's law applies; the gas concentration is proportional to the partial pressure of gas. It is also demonstrated that there is a reasonable correlation between gas solubilities and the Lennard-Jones force constants, although detailed departures from this behaviour are observed for the different materials. The results show that, in general, the presence of one gas can affect the diffusion and solubility of another, although the solubility and diffusion of carbon dioxide were found to be independent of other gases. In particular, an apparent competition is observed between nitrogen and oxygen in terms of solubility. Moreover, the nature of the interaction between gases depends on the degree of branching and the state of annealing of the polyethylene. Contrary to expectation, it is shown that annealing, whilst increasing the crystallinity, increases the permeability of all gases for the only two samples studied in this regard. (c) 2005 Elsevier Ltd. All rights reserved.