Zeolite membranes (layers of zeolite grown on porous supports) have a unique potential for selective, energy-efficient separations based on molecular size and/or shape. Separation experiments using zeolite membranes are typically interpreted with the Maxwell–Stefan model of diffusion, where the Fick diffusion coefficient can be expressed as the product of the Maxwell–Stefan diffusion coefficient and a term involving thermodynamic corrections from the adsorption isotherm. This thermodynamic factor is typically determined from adsorption isotherms collected on a powdered zeolite, as measuring adsorption isotherms on intact membranes is challenging. In this study, we present a non-destructive, reusable apparatus with which to measure adsorption isotherms of intact porous membranes and use it to test the validity of the assumption that using powder adsorption isotherms is equivalent to using membrane adsorption isotherms for the purpose of determining the thermodynamic correction factors. We find that treating a zeolite membrane as a combination of a zeolite powder and a zeolite membrane is a reasonably good approximation to the overall isotherm. The lack of hysteresis in the isotherms indicates that mesopores are not detectable, indicating that the influence of crystal intergrowth defects on membrane transport is limited to their effects on the Maxwell–Stefan diffusion coefficients. The approximation of using zeolite powders to determine the thermodynamic correction factors for zeolite membranes is, in this light, a reasonable one.