The permporometry measurements are performed with respect to a series of zeolite membranes with different defect sizes, which can be further applied for in situ measurement of the defect size distribution in zeolite membranes. Gas permeation experiments are conducted for CO2/N-2 gas mixture to test the separation performance of the studied zeolite membranes. By taking into account the "t-layer" on defect walls, a mathematical model and the corresponding procedure are developed so that the defect size distribution in zeolite membranes can be calculated by using the results of permporometry measurements. The defect size distribution and the maximal defect size show a good correlation with the separation performance of CO2/N-2 gas mixture for zeolite membranes. It is demonstrated that the separation performance of zeolite membranes is mainly determined by large defects. It has been shown that the permporometry-based methodology proposed in this contribution is an effective way for the quality evaluation of zeolite membranes. (C) 2009 Elsevier B.V. All rights reserved.