An AgMnOx catalyst (3.5 wt.% Ag) incorporating silver ions in a Mn2O3 phase exhibits high performances for soot oxidation below 300 degrees C. Its structural and redox properties have been investigated under reaction conditions using in situ XRD and DTA-TGA measurements. The catalyst appears unmodified during soot combustion experiments under oxygen, but in the absence of oxygen the soot is stoichiometrically oxidised by lattice oxygen leading to catalyst bulk reduction according to the steps Mn2O3 -> Mn3O4 -> MnO. The isotopic reaction product composition ((CO2)-O-16, (COO)-O-18-O-16, (CO2)-O-16, (CO)-O-16 and (CO)-O-18) obtained during soot combustion experiments under O-18(2) reveals that the reaction follows a redox mechanism, in which the transfer of lattice oxygen from the catalyst to the soot is responsible for the soot ignition at low temperature. (C) 2012 Elsevier B.V. All rights reserved.