Mercury oxidation by hydrochloric acid over the metal oxides supported by anatase type TiO 2 catalysts, 1 wt.% MO x /TiO 2 where M = V, Cr, Mn, Fe, Ni, Cu, and Mo, was investigated by the Hg 0 oxidation and the NO reduction measurements both in the presence and absence of NH 3 . The catalysts were characterized by BET surface area measurement and Raman spectroscopy. The metal oxides added to the catalyst were observed to disperse well on the TiO 2 surface. For all catalysts studied, the Hg 0 oxidation by hydrochloric acid was confirmed to proceed. The activity of the catalysts was found to follow the trend MoO 3 ~V 2 O 5 >Cr 2 O 3 >Mn 2 O 3 >Fe 2 O 3 >CuO>NiO. The Hg 0 oxidation activity of all catalysts was depressed considerably by adding NH 3 to the reactant stream. This suggests that the metal oxide catalysts undergo the inhibition effect by NH 3 . The activity trend of the Hg 0 oxidation in the presence of NH 3 was different from that observed in its absence. A good correlation was found between the NO reduction and the Hg 0 oxidation activities in the NH 3 present condition. The catalyst having high NO reduction activity such as V 2 O 5 /TiO 2 showed high Hg 0 oxidation activity. The result obtained in this study suggests that the oxidation of Hg 0 proceeds through the reaction mechanism, in which HCl competes for the active catalyst sites against NH 3 . NH 3 adsorption may predominate over the adsorption of HCl in the presence of NH 3 .