In this work, the interactions at low temperature (125 °C) of gaseous NH3, NO and O2 with carbon-supported manganese oxide catalysts have been extensively studied, mainly via step–response experiments, with the objective of explaining the low-temperature mechanism of the SCR reaction on such catalysts. To achieve this aim, the present work analyses the active phase and the interaction of NO species with the active phase both in the absence and in the presence of O2. The catalyst was found to be Mn3O4 composed locally of octahedral Mn2O3 and a slightly oxidised (oxygen excess) tetrahedral MnO. Both local species have different though interrelated roles during the SCR process. On the octahedral Mn2O3 NO can be adsorbed as nitrosyls (on oxygen vacancies) and as linear and bridged nitrites on oxygen atoms (absent under steady state SCR conditions). On the tetrahedral MnO NO2 is adsorbed as nitrates on hydroxyl groups previously formed by the reaction of water with the oxygen excess of this phase. NO2 is produced via the reaction of nitrosyls with O2. These nitrates are responsible for the partial deactivation of the catalyst under SCR conditions.