The photocatalytic decomposition of toluene gas was investigated using TiO 2 prepared by atomic layer deposition (ALD) on nanodiamond (ND) under UV irradiation. The photocatalytic activities of ND-supported TiO 2 catalysts were compared with that of P-25. The photocatalytic activity of P-25 was rapidly decreased with time due to the accumulation of reaction intermediates formed on the surface as a result of partial oxidation of toluene. Comparing to P-25, deactivation of ND-supported TiO 2 catalysts was less pronounced. It is suggested that ND surface facilitates total oxidation of toluene on TiO 2 and reduces poisoning of TiO 2 surface with time.