Carbon membranes are prepared by the carbonisation of a thin film of phenolic resin deposited on the inner face of an alumina tube. Air oxidative treatments at temperatures in the range of 75-350 degreesC, prior to carbonisation (pre-oxidation) or after carbonisation (post-oxidation) were tested in order to improve the separation characteristics of carbon membranes when used with hydrocarbon mixtures such as olefin,paraffin and n-butane, iso-butane. The range of selectivities obtained for the systems studied are: ethylene/ethane, 2-11; propylene, propane. 10-50: n-butane/iso-butane, 10-40. A trade-off between selectivity and permeability or permeance was observed for all systems. The composition of the hydrocarbon mixture affects the selectivity of separation and permeance. However, feed pressure has hardly any influence on separation. The modification of permeance with temperature reveals that separation takes place according to an activated mechanism. The separation of hydrocarbon molecules by the membrane seems to occur by means of a combination of molecular sieving and adsorption mechanisms. The storage of carbon membranes under a hydrocarbon environment (i.e. propylene or n-butane) does not cause any significant change in membrane performance. (C) 2002 Elsevier Science B.V. All rights reserved.