Isostructural redox-active ionic crystals were synthesized with large-molecular-size and easily reducible alpha-Dawson-type polyoxometalates [alpha-P2M18O62](6-) (M = Mo and W) as building blocks. The crystals possess large closed pores and show rapid (1 h) and large amounts (5.5 or 4.4 mol mol(-1)) of Cs+ uptake at room temperature via reduction of the polyoxometalates. The reduced compounds can be fully oxidized, and Cs+ is released within 10 min.