Post-synthesis modification of a porous ionic crystal proceeded via two steps (acid treatment followed by ion-exchange) in an aqueous solution and a single-crystal-to-single-crystal manner. Compound K-2[Cr3O(OOCH)(6)(etpy)(3)](2)[alpha-SiW12O40]center dot 8H(2)O (etpy = 4-ethylpyridine) [1a] is a porous ionic crystal with one-dimensional channels, which can accommodate guests such as water, alcohols, and halocarbons. Crystals of la were immersed in an aqueous HCl solution (acid treatment), and the etpy ligand which was exposed to the one-dimensional channel was removed and exchanged with water. The formula of the resulting compound was (etpyH(+))(2)[Cr3O(OOCH)(6)(etpy)(2)(H2O)](2)[alpha-SiW12O40]center dot 6H(2)O [2a], and K+ ions, which are potential guest binding sites, were simultaneously removed by this treatment. Reincorporation of K+ ions was attempted by immersion of 2a into an aqueous CH3COOK solution (ion-exchange), and K-2[Cr3O(OOCH)(6)(etpy)(2.5)(H2O)(0.5)](2)[alpha-SiW12O40]center dot 8H(2)O [3a] was formed. Increase in sorption capacity by the two-step post-synthesis modification was confirmed by sorption isotherms and Monte Carlo-based simulations using water as a probe molecule. The role of K+ ions as water binding sites was confirmed by water sorption isotherms of alkali metal ion-exchanged compounds.