The gain of channel electron multipliers (CEM) decreases rapidly during experiments which involve exposure of samples to ∼10−8Torr H2O during electron bombardment—an environment which very effectively oxidizes semiconductors. In subsequent experiments it was found that reoxidation of samples of CEM glass (it is subject to reduction treatment during manufacture) occurs when it is exposed to the same set of conditions. We discuss the possible influence of this on the gain degradation commonly observed as a function of total counts.