The adsorption and transformation of ammonia over V2O5, V2O5/TiO2, V2O5-WO3/TiO2 and CuO/TiO2 systems has been investigated by FT-IR spectroscopy. In all cases ammonia is first coordinated over Lewis acid sites and later undergoes hydrogen abstraction giving rise either to NH2 amide species or to its dimeric form N2H4, hydrazine. Other species, tentatively identified as imide NH, nitroxyl HNO, nitrogen anions N-2(-) and azide anions N-3(-) are further observed over CuO/TiO2. The comparison of the infrared spectra of the species arising from both NH3 and N2H4 adsorbed over CuO/TiO2 strongly suggest that N2H4 is an intermediate in NH3 oxidation over this active selective catalytic reduction (SCR) and selective catalytic oxidation (SCO) catalysts. This implies that ammonia is activated in the form of NH2 species for both SCR and SCO, and it can later dimerize. Ammonia protonation to ammonium ion is detected over V2O5-based systems, but not over CuO/TiO2, in spite of the high SCR and SCO activity of this catalyst. Consequently Bronsted acidity is not necessary for the SCR activity.