The influence of an inert gas on the separation performances of a dense polymeric membrane module working under partial vacuum on the downstream side, such as possibly encountered in gas permeation, vapor permeation or pervaporation, has been investigated through an experimental and theoretical study. A whole range of situations on the downstream side, covering ideal vacuum pumping (i.e. zero downstream pressure under leak free conditions) to inert gas sweeping under atmospheric pressure has been tested. A theoretical framework, previously developed for single permeant situation has been extended to the multicomponent case. The separation of methanol and 2-propanol by a dense silicone rubber membrane confirms the ability of this simple modelling strategy to offer quantitative predictions of the permeate composition under variable downstream pressure and inert gas flowrate conditions. Based on this observation, the implications of an inert gas contribution on pervaporation or gas separation operation are discussed, particularly in relationship to the global energy consumption of the system or to analytical devices making use of a gas sweep. (C) 2003 Elsevier Science Ltd. All rights reserved.