Moisture membranes play a key role in high performance protective clothing by preventing outside water to get in while allowing the human body to perspire properly. However, these membranes are confronted to high environmental constraints, in particular within firefighters' protective clothing. The resulting aging effect may lead to modifications of their performances, for example their mechanical or barrier properties. In this study, the thermal aging of an e-PTFE/Nomex (R) moisture membrane was carried out at five temperatures between 190 and 320 degrees C. The effect of aging on the mechanical performance was assessed by tensile tests and trapezoid tear strength measurements. Variation in the moisture membrane water vapor permeability due to aging was also studied. Large modifications in the membrane mechanical properties as a result of thermal aging were recorded. It was associated in part with a degradation of the Nomex (R) fibers. The membrane water vapor permeability was observed to decrease with aging time below 220 degrees C while values larger than those corresponding to the unaged material were measured above that temperature. This was possibly related to the occurrence of two competing phenomena relative to water vapor permeability: closure of pores in the e-PTFE laminate and creation of cracks and holes. These results show that the aging of the moisture membrane must be considered carefully while estimating the service life of protective clothing. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci 121: 3101-3110, 2011