The reduction of molecular oxygen is the counter reaction of most photocatalytic reactions proceeding oxidatively on titanium dioxide particles. We have quantitatively analyzed the reductive production of hydrogen peroxide and superoxide ion from oxygen in an aqueous solution containing 2-propanol as the scavenger of positive holes. The rates for the production of hydrogen peroxide and superoxide ion were determined by colorimetry using iodide and nitroblue tetrazolium, respectively. In addition, the oxidation of 2-propanol to acetone was monitored. Based on a comparison of these production rates, it was concluded that the main product from oxygen is hydrogen peroxide when TiO2 powder consisting mainly of anatase-form particles is used, whereas the main product is superoxide ion when TiO2 powder consisting mainly of rutile-form particles is used. The difference in the photocatalytic activity between these powders can be attributed to the difference between the reduction paths of oxygen on these powders. It was also found that the superoxide ion generated from molecular oxygen spontaneously reacts with 2-propanol to produce acetone and hydrogen peroxide. (C) 2004 Elsevier Inc. All rights reserved.