The main objective of this study is the determination and description of mass transfer mechanisms of fluids under various states-gas, liquid and supercritical-through microporous inorganic membranes. The work was focused on the permeation of CO2 through membranes whose selective layer is constituted by MFI zeolite, a material with particular properties of porosity, crystalline structure and adsorption. To describe the evolution that experiences the fluid inside the microporous medium as a function of pressure and temperature, models based on the formulation of Maxwell-Stefan were selected. As a whole, it appears whatever the state of the fluid the transfer mechanisms are the same. The transfer of fluid results from a combination of diffusive mechanism and interaction with the membrane surface. It is well in agreement with the fact that the proximity between the pore size and molecules enables one to foresee. (C) 2004 Elsevier Ltd. All rights reserved.