The adsorption of NO and NH3 on MnOx/Al2O3 catalysts, used for the low temperature selective catalytic reduction of NO, was studied separately by use of TPD (with labelled compounds) and FTIR. Besides, the influence of O-2 on the adsorption of the reactants was investigated. At 323 K, NH3 can adsorb as coordinated NH3 and ammonium ions, which both have comparable thermal stability. Hence, both of them, as well as amide species, can be present at reaction temperature (423 K). In the presence of O-2 the relative distribution of these three surface species does not change. NO adsorbs in small quantities on the surface of these catalysts after an inert treatment as Mn3+-NO nitrosyls and some nitrites/nitrates. However, it adsorbs in significant amounts after an oxidative pretreatment and in high amounts (NO/Mn approximate to 1) in the presence of gas phase O-2. At 423 K, the following compounds can be present, in increasing order of thermodynamic stability : linear nitrites, bridged nitrites, monodentate nitrites ( bridged nitrates ( bidentate nitrates. The formation of these five species is strongly enhanced in the presence of O-2, and probably proceeds by NO oxidation. In contrast, nitrosylic compounds are unstable in O-2 containing atmospheres. The uptake of NO in the presence of O-2 is lower than the NO2 uptake, and relatively more stable nitrates are formed in the latter case. The role of O-2 is to oxidise NO at the surface rather than in the gas phase.