Self-sensitisation of photo-oxygen evolution occurs in aqueous dispersions of silver zeolites. In presence of C1-, chlorine is the photoproduct in acidic medium, and the same type of self-sensitisation occurs. Self-sensitisation means that systems which are first insensitive to light of a certain wavelength become photo-active after they have been illuminated by light of higher energy. For a better understanding of silver zeolites, we have carried out EH-MO calculations on the 6-6 subunit (SBU) of a zeolite, on the 6-6 SBU with an Ag-atom in the center, on the 6-6 SBU with one Ag-atom in the center and one outside on top of the hexagon, and finally on another with one Ag-atom in the center and two Ag-atoms outside, each on top of a hexagon. The Ago in the cage of the 6-6 SBU is significantly polarized by the 6-6 SBU environment. The energy barrier to escape the 6-6 SBU is 0.8 eV for Ago and 0.5 eV for Ag' . The HOMO of the Ag(6-6 SBU) is a totally symmetric 5s* orbital and the LUMO is a 5pt type. 5pf+5s* electronic excitation reduces the energy barrier and allows an (Ago)* to exit the 6-6 SBU, provided the excited-state lifetime is long enough. The MO picture predicts low-energy charge-transfer transitions from the zeolite framework to the 5s* orbital. The highest occupied orbitals of the zeolite framework are localized on the 0-atoms. Interactions between an Ag-atom in the 6-6 SBU and one or two external Ag-atoms are discussed.