An in situ nondestructive characterization method for zeolite composite membranes, based on transient permeation experiments, is proposed in the present article. This technique allows an accurate evaluation of the zeolite selective layer thickness as well as the thermodynamic and transport properties of sorbing hydrocarbons. To this end, a dynamic model representing mass transport phenomena within the whole permeating module is derived. In order to get the crystal layer properties, a two-step experimental approach is required. The zeolite layer effective thickness is first estimated with the permeation of a nonadsorbing species like hydrogen. Then, the adsorption equilibrium constants and the diffusion coefficients of butane isomers are determined. Three membranes obtained by different synthesis procedures are studied in the Henry domain. The estimated values of the equilibrium constants and diffusion coefficients for normal butane and isobutane are favorably compared with existing literature data. (C) 2008 American Institute of Chemical Engineers.