The use of CdSe-ZnS type I quantum dots as redox photocatalysts with visible light in water is investigated by monitoring the oxidation reaction of 8-oxo-2鈥?deoxyguanosine and the reduction of nitrophenylalanine derivatives. The detection of reaction intermediates and the identification and quantitation of reaction products establish that both reactions are photocatalyzed simultaneously by CdSe-ZnS type I quantum dots. The photocatalyzed reactions are observed without the need for sacrificial reactants and with high turnover numbers (>400). These results demonstrate that CdSe-ZnS type I quantum dots can work efficiently as redox photocatalysts in water solution.