A mathematical model has been developed to predict mass and heat transfer during the steam generation process in a novel zeolite-water adsorption heat pump system. This model features a three-phase calculation and a moving water-gas interface. The calculations were carried out in the zeolite-water and zeolite-gas regions. An enthalpy form is adopted to account for water evaporation. Model outputs are compared with experimental results for validation. The thermal response in the reactor is well simulated with a relative error between the calculated and experimental steam mass of 0.6% to 1.9%. The effects of operational temperatures on the steam mass are also investigated numerically. Raising the water temperature rather than zeolite temperature enhances the mass of steam produced. The peak temperature in the reactor increases from 300 to 311 degrees C as initial zeolite temperature rises from 80 to 120 degrees C.