The effect of adsorption of water onto NaMOR (Na+-type mordenite) and HMORs(H+-type mordenites) with various SiO2/Al2O3 ratios on microwave heating was investigated. The adsorption of water markedly influenced the microwave heating of NaMOR with steady feeding of water vapor. For example, in the presence of water vapor, NaMOR was heated more rapidly and to higher temperature (432 K at highest) than under a dry condition. On the other hand, the adsorption showed little effect of water on the microwave heating of HMORs. A TPD experiment indicated that water was more strongly adsorbed on NaMOR than on HMORs. These results show that the strong interaction of water with NaMOR improves the heating rate and the temperature reached in the microwave heating process.