Three kinds of ZSM-5 (MFI structure) zeolites [SiO2/Al2O3 = 23.8 (Na form), 39.0 (NH4 form) and 193(NH4 form)] were alkali-treated, and the changes in structural and water adsorptive properties were investigated. With the alkali treatment, the MFI structure was preserved; a siliceous species was selectively dissolved from the framework of a zeolite, and mesopores were created in the zeolites. At the same time, NH4-form ZSM-5 zeolites (SiO2/Al2O3 = 39.0 and 193) were ion-exchanged to form Na in an aqueous solution of NaOH. In particular, for ZSM-5 (SiO2/Al2O3 = 23.8), the water adsorption changed with the alkali treatment. The difference in the amount of adsorbed water on the zeolite between 30 degrees C (adsorption temperature) and 100 degrees C (desorption temperature) increased by the treatment. The desorption temperature of water adsorbed at the stronger site decreased from 106 to 72 degrees C with the alkali treatment. These results indicate that the introduction of mesopores in ZSM-5 contributed to the enhanced water adsorption.