Photocatalytic decomposition of ethanol and acetaldehyde in water containing dissolved oxygen was studied using a microelectrode technique. We employed a partially Pd-covered TiO2 film as a model for the metal-deposited photocatalyst and observed concentration changes of dissolved oxygen above the TiO2 surface due to the radical chain oxidation initiated by the photooxidation. The concentration change of dissolved oxygen was larger in acetaldehyde-containing aqueous solution than in ethanol-containing aqueous solution. In addition, we examined the role of superoxide radicals in the decomposition of ethanol and acetaldehyde using a biological enzyme, superoxide dismutase (SOD). The amount of oxygen consumed in the ethanol solution was much less in the presence of SOD than in the absence of SOD. However, the amount consumed in the acetaldehyde solution was greater in the presence of SOD than in the absence of SOD. Based on these results, we have suggested decomposition mechanisms for ethanol and acetaldehyde.