Carbon molecular sieving membranes offer many advantages for difficult gas separations. The selectivities achieved with these membranes are much higher than those typically found with polymeric materials, without sacrificing productivity. While they perform very well with high-purity, dry feeds, the work in this paper will, show that they are vulnerable to adverse effects from exposure to water vapor. Tests made with humidified air feeds ranging from 23 to 85% relative humidity show that some performance losses occur at all levels, with the losses increasing as the humidity level increases. In addition, most of the performance loss occurs in the initial period of exposure to water vapor. Despite this vulnerability, it was observed that significant membrane function was maintained at the lower levels of exposure. The effects of other factors, including feed pressure, membrane orientation, and the nature of the carbon itself are also considered.