Appropriately prepared Ag + -A zeolites evolve oxygen on illumination in the presence of water. Cl - occluded in the cavities of zeolite A forms silver chloride complexes which are essential for the photochemical behaviour of the system. At approximately neutral pH, Cl - leads to enhanced O 2 evolution up to a critical concentration above which the oxygen evolution drops abruptly. Cyclic voltammograms of zeolite A monograin layers on glassy carbon electrodes containing occluded silver chloride complexes in the dark and on irradiation are reported.