Physical or chemical properties of hydrated ions confined in solid nanospaces are not substantially understood, although they are indispensable for the development of new science and technology. We study the local structure around a Cu2+ of copper acetate restricted in slit-shaped micropore of two kinds of activated carbon fibers (ACFs) by X-ray absorption fine structure (XAFS) technique. The results indicate that the highly dispersed copper acetate on the surface of ACFs can be partially dissolved after the adsorption of water. Also, when we adsorb water to copper acetate-deposited ACFs, the Cu-Cu distance of the dinuclear species of copper acetate becomes longer with the ordering of the Cu-Cu bond because of the reduction of the number of the bridging carboxylate inside the micropore. In addition, the results of EXAFS spectra suggest that water molecules can play an important role for the ordering of the specific dinuclear complex of copper acetate formed in the micropore even though the number of bridging carboxylate which can stabilize the dinuclear structure is decreased by the adsorption of water. (C) 2011 Elsevier Inc. All rights reserved.