Methane adsorption on Ga ion-exchanged ZSM-5 (Ga-ZSM-5) and In ion-exchanged ZSM-5 (In-ZSM-5) was investigated by temperature-pro rammed desorption in ultra high vacuum (UHV-TPD) to clarify the difference in the influence of water vapor on the selective catalytic reduction of NOx by methane. Methane was adsorbed on Ga-ZSM-5 and In-ZSM-5, and the adsorption heats of methane were estimated to be - 98 kJ/mol and - 132 kJ/mol for Ga-ZSM-5 and In-ZSM-5, respectively. The amount of methane adsorbed on Ga-ZSM-5 decreased considerably after the preadsorption of water, while little decrease was observed on In-ZSM-5. These results suggest that one of the reasons that the suppression of HC-SCR activity by water vapor is less severe on In-ZSM-5 than on Ga-ZSM-5, may be that competitive water adsorption is much more inhibiting to methane adsorption on Ga-ZSM-5 than it is on In-ZSM-5.