The quenching rate of triplet benzophenone in water and/or water mixtures has been determined employing vitamin C, vitamin E, cystine, cysteine, reduced and oxidized glutathione, methionine and DL-penicillamine. In these systems, the ketyl radical quantum yield and the benzophenone photoreduction yield have also been measured. The ketyl quantum yield is 1 .0 in presence of vitamin C and smaller than 0.3 in presence of glutathione, cysteine and cystine. The data imply that quenching by thiols and disulfides takes place, at least in very polar solvents, mainly by a mechanism involving charge transfer intermediates.