Increasingly stringent ambient air quality standards coupled with the need to improve fuel economy has drawn significant attention to the search for emission control systems for lean burn engine vehicles. Much of the focus has been on zeolite-based catalysts for die conversion of NOx to N-2 for automotive exhaust emission control. Under certain conditions, these catalysts are highly active catalysts for the reduction of NOx using hydrocarbons as the reductant. However, many of these catalysts suffer from a variety of deactivation processes such as irreversible poisoning by SOx or hydrothermal dealumination. In addition to these deactivation processes, a recent focus of our research has been on the influence of water vapor on the activity of zeolite-bused catalysts at low operating temperatures. We observe a hysteresis in catalytic activity of lean NOx reduction (NO feed concentrations <100 ppm) upon increasing and decreasing temperature ramps at the low end of the operating window, that being from 100 to 300 degreesC using hydrocarbons as reductants. We describe these reversible influences of water vapor and the implications for this hysteresis in catalytic activity for the application of zeolite-based catalysts in lean NOx catalysis, and compare these results to the instance of using ammonia as reductant.