In gas-liquid membrane contacting, it is important to know the gas permeance of microporous hydrophobic membranes used in such a system. Gas permeance of carbon dioxide from a CO2-N-2 mixture having a low CO2 concentration into an aqueous KOH solution through flat microporous (Celgard 2400, Saint-Gobain R128-10)/nonporous poly(1-trimethysilyl-1-propyne) (PTMSP) membranes is therefore studied at zero net total pressure difference (DeltaP = 0). Pure gas permeance data of CO, through the same membranes for positive DeltaP and gas-gas system are extrapolated to zero mean pressure (P = 0) to find also gas permeance. Conventional theoretical estimates of the liquid film resistance for such systems are compared with the experimental results for the liquid film resistance; they were found to be considerably higher than that estimated from the theory based on a liquid film having a fast chemical reaction. The membrane resistance obtained by subtraction of the experimental liquid film resistance from the total resistance of the system appears to predict the CO2 permeance for the thicker PTMSP film measured under positive DeltaP quite well. However, this method leads to higher estimates of membrane resistance for thinner PTMSP films, Celgard 2400 and other supposedly highly permeable porous substrates compared to those based on the data obtained by extrapolation to P = 0. There appears to be an upper limit of permeance which may be determined correctly in such experimental measurements based on DeltaP = 0. This upper limit is considerably higher than what has been achieved by earlier investigators. Several factors potentially contributing to this discrepancy have been pointed out. (C) 2004 Elsevier B.V. All rights reserved.