The reactivity in NO reduction by C3H6 and O-2 Of three catalysts based on Pt supported on alumina, titania and titania-alumina is studied and compared with the nature and stability of NOx adspecies in these catalysts as determined by NO thermodesorption studies and in situ diffuse reflectance infrared (DRIFT) experiments. A relationship between activity of the catalyst and amount of weakly and reversibly coordinated NO species (suggested to be NO reversibly coordinated to surface oxygens generated by spillover on the noble metal, but stabilized by support sites) is indicated. It is also shown that the catalytic activity depends in part on the rate of transformation (in the presence of gaseous O-2) of this weakly coordinated NOx species to species which decompose at higher temperatures (above 300degreesC) and with which the formation of NO2 during catalytic tests is associated. The catalyst based on titania-alumina shows high selectivity in NO reduction together with good activity and the widest range of temperature of maximum activity, probably due to the coexistence of nanodomains of the two oxides on the surface. (C) 2003 Elsevier Science B.V. All rights reserved.