The kinetics of the deep photocatalytic oxidation of acetone was studied, Special attention was paid to the thermal deactivation process, The rate of the reaction reaches a peak at about 100 degrees C. With increasing concentration of water vapor the peak shifts towards higher temperature, whereas with increasing acetone concentration the peak shifts towards lower temperature. An increase in water vapor concentration slightly decreases the rate of oxidation at 40 degrees C and increases it at temperatures above the rate maximum, The presence of two types of acetone adsorption sites was deduced from two nu(C=O) bands of adsorbed acetone at 1683 and 1710 cm(-1) and can explain the dependence of the rate on acetone concentration. The rate of oxidation increases at temperatures below the peak, mainly because of the increase in the rate coefficient at the sites of the first type. This behavior has been ascribed to the thermal oxidation of an intermediate of the photocatalytic process. Abrupt drop in rate at temperatures above the peak is due to the partial oxidation products of acetone thermal oxidation that modify the TiO2 surface.