Water-oxide interactions play an important role in chemical, environmental and biological systems. Even though these systems have been extensively studied, the nature of the water-oxide interface remains poorly understood. Due to its simplicity and environmental relevance, the MgO(100) surface has become a prototype for studying such interactions. However, basic issues such as molecular versus dissociative adsorption at the H2O/MgO(100) interface have not been resolved. We have utilized Ambient Pressure Photoemission Spectroscopy at beamline 11.0.2 of the Advanced Light Source to address such questions. This unique experimental set-up is capable of performing photoemission experiments in the Torr pressure range allowing measurements of the H2O/MgO(100) interface under equilibrium conditions. We will present results relating to the degree of hydroxylation and uptake of H2O on MgO(100) thin films grown on Ag(100) at room temperature and up to 1 torr of H2O pressure. Furthr, we have studied the influence of film thickness on the reactivity of the MgO(100) film using NO2 as a probe molecule. Our measurements show that films with thicknesses greater than 5 monolayers behave similar in nature to bulk MgO.