To elucidate the interaction mechanism of NOx (NO2 and NO) and H2O on the surface of H-type mordenite (HM), the dynamic adsorption properties of NO2 + H2O + O-2 and NO + H2O + O-2 mixtures were examined and the surface reaction of NOx and H2O was studied by an in-situ FT-IR spectroscopic method. It was found that NO2 can be dissociatively adsorbed on two kinds of adsorption sites on HM to form nitric acid like species by interacting with Bronsted acid sites, whereas NO + O-2 + H2O adsorption is a competitive process of NO oxidation and H2O adsorption without leading to formation of nitric acid like species. The difference between NO2 adsorption sites and NO oxidation sites gives rise to the difference in their breakthrough properties. Adsorption/catalytic reaction models are proposed based on column adsorption and FT-IR results.