The adsorption of nitric oxide (NO) by ammonia over a series of V 2 O 5 /TiO 2 catalysts used for selective catalytic reduction (SCR) has been investigated by solid-state 15 N NMR. The chemistry and the surface acidity are found to vary dramatically as the weight loading of vanadia is increased from 0 to 10%. NO reacts over evacuated TiO 2 and V 2 O 5 /TiO 2 at room temperature to form nitrous oxide (N 2 O). Over the V 2 O 5 /TiO 2 catalysts N 2 is also produced upon room temperature adsorption. The chemical shift of the terminal nitrogen of N 2 O adsorbed on the catalyst is an indicator of the Lewis acid character of the surface. The Lewis acidity of the surface is found to decrease with the loading of V 2 O 5 on the titania support. Ammonia adsorbs in two forms on evacuated TiO 2 and V 2 O 5 /TiO 2 . These two forms differ in their surface mobility. The reaction of NO with NH 3 over the catalysts proceeds to a measurable degree at room temperature. N 2 is the predominant product; however, N 2 O is also produced by the partial reduction of NO. Following preadsorption of ammonia, the chemical shift of the terminal nitrogen of N 2 O is similar to that of samples with a high V 2 O 5 loading regardless of the actual vanadia loading. This indicates that ammonia is adsorbed predominately on the Lewis acid sites of the catalyst, blocking the access of N 2 O to these sites.