The effect of water vapor concentration on the reaction rates of NH3 oxidation and N2O decomposition catalyzed by calcined limestone was investigated by use of a fixed bed reactor under atmospheric fluidized bed combustion conditions. Both reactions were inhibited by the presence of water vapor. Inhibition of NH3 oxidation by H2O could be expressed as a decrease in the number of available active sites according to Langmuir-type adsorption of H2O on one kind of active site. Inhibition of N2O decomposition by H2O could be approximated by assuming two kinds of active sites, both of which adsorb H2O according to Langmuir-type adsorption equilibrium, but the adsorption equilibrium constant was different between two types of active sites. The equilibrium constant of H2O adsorption on active sites for NH3 oxidation was found to be different from those of H2O adsorption on active sites for N2O decomposition.