The effect of the surface acidity of modified TiO2 samples on the rate of photocatalytic decomposition of acetone vapor was studied. The TiO2 activity was shown to depend strongly on the concentration of acid and basic sites on the surface. It was 1.23 times greater for TiO2 treated with 10 M H2SO4 solution than for the untreated sample. This dependence may be related to changes in the adsorption energy of the reagents.