WE have succeeded in preparing ion-permselective membranes from a sheet of commercial polyethylene which are cation-selective on part of their surface and anion-selective on the adjoining part with a sharp line of demarcation between them1. To this end we treated the original polyethylene film either with sulphuryl chloride in the liquid or vapour state or in a mixture of chlorine and sulphur dioxide gas, in the light of incandescent lamps. Swelling by addition of carbon tetrachloride enhanced the reaction. A film, 0.1 mm thick, reacted completely in 2–3 h at about 50° C. This sulphochloride film was washed in carbon tetrachloride, dried, and half of it was pressed between two sheets of rubber with two metal plates. The rest of the film projected from this device and was immersed in a bath of tetraethylene pentamine at 80° C for 2 h. It became yellow and was transformed into an anion-selective membrane: the pentamine molecules became linked by sulphamide bonds into the film. The device was then opened and the whole film immersed into a 5 per cent solution of sodium dihydroxide in 1: 1 dioxane/water. In this bath all sulphochloride groups which had been protected from amination, between the rubber sheets underwent hydrolysis to sulphonate groups whereas the aminated part of the film was not disturbed. The sulphonated portion of the membrane proved to be strongly cation-selective, having 95 per cent permselectivity and a resistance of 2 ohms/cm2. The aminated portion was anion-selective with 87 per cent permselectivity, but improved after methylation with alcoholic methyl bromide to 96 per cent, its resistance being 1 ohm/cm2.