Quantum yields of chloride ions exceeding unity are obtained upon illumination of TiO2 layers in aqueous solutions containing CCl4 and methanol. The TiO2 layers were prepared by spin coating using concentrated colloidal solution (prepared by hydrolysis of the propoxide). Absorbed light intensities were 7 x 10(-11) to 2 x 10(-8) ein cm(-2) s(-1). There is only a small, if any, effect of methanol concentration on the Cl-yield in the range 0.2-5 M. Above 5 M, the yield remains nearly constant in the absence of air but decreases in aerated solutions. Only negligible Cl- yield is obtained in the absence of methanol. In the presence of oxygen, the rate of photocatalytic Cl- build up is proportional to the square root of the light intensity, as expected in a chain reaction. A similar study in oxygen-free solutions shows [Cl-] leveling off at the higher light intensities. The quantum yield increases with pH in both the presence and absence of oxygen, reaching values phi approximate to 7 at pH 12.2, at the lowest light intensity. In the absence of oxygen, there is no observable effect of CCl4 concentration on the Cl- yield above I x 10(-3) M, while the oxygen-containing systems show nearly linear increase of the yield upon increasing [CCl4]. Thermal catalyzed formation of chloride is observed in the absence of oxygen. In the absence of oxygen, removal of adsorbed hydroxyl radicals, OHads., by the methanol enables the electrons that have escaped recombination to react with CCl4, producing chloride ions and CCl3. radicals. This is followed by electron injection to the TiO2 conduction band and subsequent hydrolysis of the CCl3+ intermediate to carbon dioxide and HCl. A chain reaction mechanism is proposed also for oxygen-containing systems. The CCl4 - O-2(.-) adduct is the chain carrier and the termination involves dismutation of O-2(.-) radical ions.