The effect of high H2O concentration during oxy-steam combustion on the oxidation of methane and ammonia was investigated both experimentally and numerically. Comparison experiments between O-2/N-2 and O-2/H2O atmosphere were performed in a flow reactor at atmospheric pressure covering fuel-rich to fuel-lean equivalence ratios and temperatures from 973 to 1773 K. Experimental results showed that the presence of high H2O concentration dramatically suppressed CO formation at temperatures above 1300 K. High H2O concentrations inhibited NO formation under stoichiometric and fuel-lean conditions but enhanced NO formation under fuel-rich conditions. The chemical kinetic mechanism, which was hierarchically structured and updated, satisfactorily reproduced the main characteristics of CO and NO formation. High H2O concentrations significantly alter the structure of radical pool and subsequently the formation of CO and NO. Ultralow CO concentrations above 1300 K are attributed to the enhancement of CO + OH reversible arrow CO2 + H by high OH radical concentrations. NO suppressions under stoichiometric and fuel-lean conditions are caused by strong suppression Of NH2 + O reversible arrow H + HNO in the pathway NH2 -> HNO -> NO. This suppression is due to the lack of 0 radicals. By contrast, NO enhancement under fuel-rich conditions is caused by the significant enhancement of NH2 + OH reversible arrow NH + H2O in the pathway NH2 -> NH -> HNO -> NO. This enhancement is due to the fairly high OH concentration in the O-2/H2O atmosphere.