Silica-supported MoO3 catalysts with MoO3 loadings up to 31% w/w were prepared, characterized and tested in the selective catalytic oxidation (SCO) of ammonia to N-2 under dilute conditions. It is found that the catalysts are active and selective in the reaction, and that the catalytic performance increases on increasing the Mo loading. Crystalline MoO3, detected over the silica support, is supposed to be the active species in the reaction. The reactivity of the catalysts is depressed by water addition to the feed at low temperatures and is enhanced by the presence of selected promoters, like Bi and Ph.