Permeation porometry, aka "permporometry" is used as a toot to investigate the connected pore size distribution of asymmetric meso-porous membranes. The method is based on the selective filling of specific sized pores with a condensable vapor, and the analysis of a permeating non-condensable gaseous probe through the remaining open pores. The technique is frequently proposed as a convenient means of detecting pinholes in supported membranes. However, artifacts are found to arise in the desorption isotherm due to a combination of chemical diffusion of the probe gas through the pore-blocking liquid, cyclohexane, and the desorption of cyclohexane from the support. These artifacts can be misinterpreted as pinholes and/or overwhelm the contributions of real pinholes. gamma-Alumina membranes of various thickness, supported on smooth homogeneous alpha-Al2O3 compacts, are used to demonstrate the occurrence of these artifacts. Non-condensable probe gasses 02 and Ne are used to identify the crucial difference between the Knudsen and chemical diffusion of the probe through either pinholes or a condensate filled membrane. In addition it is shown how analysis of chemical diffusion permeance through the membrane can be used to obtain the membrane's tortuosity for chemical diffusion. (C) 2007 Elsevier B.V. All rights reserved.