A definitive experimental investigation has been performed to determine the transfer characteristics of a new type of mass exchanger. The exchanger was designed to perform the dual functions of transferring water vapor across its walls with a high degree of effectiveness while preventing the transport of other gaseous species. The heart of the exchanger is a novel permeable material synthesized by depositing an extremely thin polymer film atop a sheet of a common mass transferring polymer substrate. It was demonstrated that the synthesized composite-membrane sheets could be fashioned into modules and stacked to form a cross-flow mass exchanger. Two sets of experiments were performed to investigate the mass transfer characteristics of the exchanger for water vapor transfer. The first set of experiments was performed in a special highly and precisely instrumented, dual-stream, wind-tunnel test facility. On the other hand, the second set of experiments was carried out as a simulated field test. The results of these experiments revealed that mass transfer effectivenesses for water vapor as high as 50% could be readily obtained. To investigate the capabilities of the composite membrane to block the transfer of gaseous species other than water vapor, special experiments involving carbon dioxide were performed. These experiments decisively demonstrated that virtually no CO, could pass through the composite-membrane wall. The selectivity, which is the ratio of the effectiveness for water vapor transfer to the effectiveness of CO2 transfer, was found to lie in the range between 21 and 61.