Determining membrane sorption selectivity by conventional methods is usually fairly time-consuming and can even become a rather difficult task for membrane materials with low sorption levels. In this case, the amount of the species absorbed by the membrane is indeed so low that large amounts of membrane materials are usually required to determine the composition of the sorbed mixture by a common desorption method with several associated drawbacks (e.g. experimental uncertainty, long sorption and desorption times). The paper describes how performing desorption experiments directly in a H-1 NMR tube can overcome all the former limitations. The sorption selectivity of new poly (urethane-imide) membranes containing increasing contents in tertiary amine groups was determined for the azeotropic mixture ethanol (20 wt%)/ethyl-tert-butyl ether ETBE. Membrane selectivity values given by this method were very close to those obtained by the most common former method, with the advantages of being both very simple and Much quicker to implement. (c) 2006 Elsevier B.V. All rights reserved.