A study has been carried out of the interactions of NH3 with TiO2, Nb2O5, and Nb2O5/TiO2. Raman spectroscopy was used to characterize the structure of the adsorbed NH3 and perturbations of species present at the surface of the adsorbent. On each oxide, NH3 adsorbs predominantly at Lewis acid sites. Hydrogen-bonding occurs between the adsorbed NH3 and OH groups present on the surface of TiO2. A small concentration of NH 4 + is observed, consistent with the relatively low concentration of Bronsted acid sites compared to Lewis acid sites on each of the samples investigated. Exposure of Nb2O5/TiO2 to NH3 at temperatures up to 500°C does not result in partial reduction of the supported niobia.