A systematic study of water adsorption isotherms of zeolite 3A and 4A crystals over a pressure range of 10(-5) to 10(-1) bar and temperatures from 25 to 250 degrees C was carried out. This work focuses on water isotherms of 3A and 4A zeolite crystals, not pelletized zeolites containing binders. Zeolite 3A crystals show approximately 12% less swing working capacity for partial pressure swing processes at room temperature for deep dehydration, when compared to 4A crystals. However, the 3A crystal can be regenerated at a lower temperature than the 4A crystal. The 4A crystal has a steeper water isotherm and higher heat of adsorption at zero coverage than 3A. Our results on 3A and 4A crystals are compared to literature data for pelletized 3A and 4A zeolites which contain binders. A new triple-site Langmuir model has been proposed and provides a satisfactory description when compared to other models, such as the Toth model and dual-site Langmuir model. The triple-site Langmuir model is consistent with the Linde-Type A crystal having three different sites for water adsorption. The improvement in fitting is significant in the region where water loadings approach a maximum. This new model was also successfully applied to existing literature data sets for the water isotherm at multiple temperatures for 3A and 4A beads. These results provide fundamental water isotherms and a simple model to evaluate 3A and 4A in deep dehydration with temperature-swing, or partial pressure-swing adsorption processes, or their combination using nontraditional monolith contactors.