This paper describes a simplified model of the diffusion process based upon a linear idealization of Fick's law. Ion permeation through polyethylene films was investigated using a conductometric technique at 28 degrees C. The electrolytes studied were KCl, NaCl, LiCl, MgCl2, CaCl2 and AlCl3. The initial concentration gradient was established by introducing 0.01 mol dm(-3) of electrolyte on one side of the membrane and deionized water on the other side. The results show that the PE membranes have a negative surface charge and exhibit selective permeability to the various cations, with very high selective affinity for Li+. The selectivity sequence was in the order K+ < Na+ < Li+ for the monovalent cations and Al3+ < Ca2+ < Mg2+ for the multivalent ions, suggesting dependence on the crystallographic, rather than hydrated radius of the cations. Permeation rates calculated from the evaluated transport parameters using a mathematical model based on Fick's law showed good agreement with the experimental values.