Kinetic study of CO oxidation in combination with experiments of temperature-programmed oxidation (TPO) and reduction (TPR) have been performed on various unsupported crystalline manganese oxides (MnOx); while the reactivity shows an order of MnO <= MnO2 <= Mn2O3 in a mixture of unit ratio Of O-2/CO at/below 523 K. We propose that under the current conditions the interaction of adsorbed CO and O is mainly responsible for CO2 formation on Mn2O3 and MnO2 catalysts, following either the Langmuir-Hinshellwood mechanism or Eley-Rideal mechanism. Meanwhile, direct evidence from transient CO oxidation suggests that the Mars-van-Krevelen mechanism may occur for all catalysts simultaneously, especially, it is predominant for the MnO catalyst. The evidence of structural modifications during reaction was confirmed by Raman spectra obtained from used MnO. (C) 2007 Elsevier B.V. All rights reserved.