Diffuse reflectance infrared fourier transform spectroscopy (DRIFTS) has been used to study NH3 and NO adsorption over a 15% w/w vanadia/titania catalyst. NH3 is adsorbed as coordinate NH3 and NH4+ species over the oxidised catalyst, leading to the reduction of the vanadia surface. At 300 degreesC, adsorbed nitrosyls species are detected, suggesting that the oxidation of gaseous or adsorbed ammonia species takes place over the V=O sites. Coadsorption experiments show that NO is able to reoxidise about the 57% of the reduced V=O groups, resulting in N-2, according to a NO + V-square --> 1/2N(2) + V=O reaction. On the other hand, NO is only adsorbed over vanadia reduced surfaces. The measure of the area of the 2 nu (V=O) bands results in an estimate of the oxidation state of vanadium. From this estimate it can be concluded that nitrosyls species are adsorbed on the catalyst surface for vanadium atoms having an oxidation state ranging from +4 to +3.1. (C) 2001 Elsevier Science B.V. All rights reserved.