The mechanism of the selective catalytic reduction of NO with NH3 over low loaded MnOx/Al2O3 catalysts has been investigated using TPRD, FTIR, and step-response studies with labelled components. Attention has been focused on the reactivity of the adsorbed NO and NH3 complexes, identified in the preceding paper, at reaction temperatures below 473 K. FTIR shows that the reaction starts with adsorption of NH3 on Lewis acid Mn3+ sites and its subsequent transformation to NH2 species; NH4+ ions, formed at Bronsted acid sites, hardly participate in the reaction. The NH2 species can react with both gas phase NO (Eley-Rideal mechanism) and reactive nitrite intermediates (Langmuir-Hinshelwood mechanism). Bridged nitrites and monodentate nitrites are able to react at these low temperatures. The role of O-2 cannot simply be explained by reoxidation of the surface to close the catalytic cycle; O-2 appears to be important both for H-abstraction from adsorbed NH3 and in assisting in the formation of reactive nitrites. Bidentate nitrates are formed at a slow rate in a NH3/NO/O-2 mixture and partly deactivate the catalysts at temperatures below 500 K. In contrast, they serve as reactive intermediates for the SCR reaction at higher temperatures. On the basis of these results, a reaction scheme in elementary steps is proposed for this type of catalyst.