Over the past decade, semi permeable membranes that transfer heat and water vapor have been investigated and applied in air-to-air or liquid-to-air energy exchangers. The water vapor diffusion resistance and modulus of elasticity are two of the most important properties of membranes. These properties play significant roles on the design and performance of the membrane based energy exchangers. In this study, measurement test methods for membrane properties are presented and important factors impacting the test results are evaluated using the test data. It is found that operating conditions and crystals or deposits in membranes influence the vapor diffusion resistances. The elastic modulus can be determined from the elastic deflection of a simply supported membrane subject to a normal pressure difference. Deflections of membranes are known to cause mai-distribution of flows in energy exchangers and this degrades their performance. Some effective methods to reduce deflections of membranes are presented. (C) 2013 Elsevier B.V. All rights rcserved