The characteristics and mechanisms of CO formation in O-2/CO2 and O-2/H2O atmospheres were investigated both experimentally and numerically. Comparison experiments in O-2/N-2, O-2/CO2 and O-2/H2O atmospheres were performed in a flow reactor at atmospheric pressure covering fuel-rich to fuel-lean equivalence ratios and temperatures from 973 K to 1773 K. Experimental results demonstrated that CO formation in an O-2/CO2 atmosphere is the highest and that CO formation is the lowest under all fuel-rich, stoichiometric and fuel-lean conditions. The updated chemical kinetic mechanism satisfactorily reproduced the experimental results. For O-2/CO2 atmospheres, the presence of a high CO2 concentration enhances CO2 + H = CO + OH and CH2(S) + CO2 = CH2O + CO dramatically, strengthens HCO + M = H + CO + M by the chaperone effect of CO2, and contributes exclusively to CH3OCO = CH3O + CO. The contribution of the pathway CO2 -> CO is significant, and CH3 -> CH3OCO -> CH2O and CH3 -> CH3OCO -> CO are exclusive channels in O-2/CO2 atmospheres. For O-2/H2O atmospheres, although the high chaperone effect of H2O facilitates HCO + M = H + CO + M, CO + OH = CO2 + H is enhanced due to the abundant OH radicals and HCCO + H = CH2(S) + CO is suppressed due to the lack of H radicals. The pathway CO >CO2 is enhanced due to sufficient OH radicals, and CH3 -> (CH2(S)) -> CH3OH -> CH3O -> CH2O and CH3 -> (CH2(S)) CH3OH CH2OH are exclusive channels. Moreover, the pathway CH2O -> HCO -> CO is amplified in both O-2/CO2 and O-2/H2O atmospheres. (C) 2017 Elsevier Ltd. All rights reserved.