Herein, we report a study of Ag3PO4 microcrystals in the accomplishment of their enhanced degradation process and bactericidal activity. Based on experimental results and density functional theory free energy profiles, we propose a new mechanism for the multifunctional competence of the Ag3PO4(110) surface. Coadsorbed H2O and O2 molecules regulate an energetically favorable pathway that efficiently activates the dissociation of H2O and stabilizes the formed reactive oxygen species (ROS) precursors: hydroxyl ( OH) and superoxide ( O2-) radicals. This work is a proof of concept to interpret the surface reactions on Ag3PO4 and provides a new perspective to understand at the atomic level the catalytic process/mechanism for the initial stages of ROS production on metal oxide semiconductor surfaces.