An in situ diffuse reflectance FTIR spectroscopy (DRIFTS) study of the selective catalytic reduction (SCR) of NO with NH3 in the presence of O-2 has been carried out over vanadia/alumina and vanadia/lanthanide-doped alumina catalysts. The SCR reaction data may be interpreted on the basis of an Eley-Rideal type mechanism between gaseous NO and adsorbed ammonia species (NH4+ and/or coordinated NH3). Although both adsorbed ammonia species seems to be reactive, NH4+ groups are more effective in the SCR reaction. Thus, at the temperature of the highest NO conversion (T-max), ammonia is adsorbed preferentially as NH4+ over the undoped catalyst, meanwhile over lanthanide-doped ones, the adsorption of coordinated NH3 becomes predominant, thus explaining the lower SCR activity of such type of catalysts. At lower temperatures, NO is adsorbed as nitrates and the NH3 adsorption as coordinated NH3 species is enhanced. These two factors drive the activity decrease.