Catalyst poisoning in tubular reactors is a common problem in many chemical processes using a variety of catalysts including zeolites and transition metals. It was observed in a plant operation that a trace impurity, ammonia found in ppm level, decreases the active life of the mordenite catalyst bed considerably. A mathematical model that describes the reversible deactivation phenomena can help manage the plant and optimize reactor throughput. In this work, a detailed dynamic model is presented that was used to describe the activation and deactivation cycles due to NH3 adsorption/desorption in a catalytic reactor used for an exothermic hydrolysis reaction. As NH3 adsorbs, poisoning the catalyst, the peak temperature moves down the bed. Eventually, the overall conversion drops significantly, and the catalyst bed must be regenerated by steam stripping or changing feed conditions to increase bed temperature. The model shows the influence of NH3 adsorption on catalyst performance in a shell and tube reactor is significant with 100 ppm of NH3 but manageable at 30 ppm, consistent with plant observations for this system. Moreover, the model suggests that an adiabatic reactor design will be much more robust to varying NH3 levels in the feed than the existing multitubular reactor.