An apparatus and methodology, were developed to measure the oxygen and carbon dioxide permeance of hollow fiber membranes (HFMs) in a gas-liquid environment. The gas-liquid permeance measurements were compared with measurements made in a gas-gas environment to validate the methodology, and to determine whether the permeance of an HFM in a gas-liquid environment differs inherently than in a gas-gas environment. The methodology is based on measuring the overall system permeance for a range of liquid velocities and isolating the membrane resistance to gas flux from the liquid boundary layer resistance by regression of a nonlinear system model to the data. The success of the methodology in accurately! estimating membrane permeance depended on the maximum liquid velocity which the system could consistently generate past the fibers.