Copper hexacyanoferrate, Cu-II[Fe-III(CN)(6)](2/3)center dot nH(2)O, was synthesized, and varied amounts of IC ions were inserted via reduction by K2S2O3 (aq). Ideally, the reaction can be written as Cu-II[Fe-III(CN)(6)](2/3)-nH(2)O + 2x/3K(+) + 2x/3e(-)K(+) <-> K-2x/3 Cu-II[Fe-x(II).Fe-1-x(II),(CN)(6)](2/3)-nH(2)O. Infrared, Raman, and Mossbauer spectroscopy studies show that Fe-II is continuously reduced to Fell with increasing x, accompanied by a decrease of the a-axis of the cubic Fn (3) over barm unit cell. Elemental analysis of K by inductively coupled plasma shows that the insertion only begins when a significant fraction similar to 10% of the Fe-III, has already been reduced. Thermogravimetric analysis shows a fast exchange of water with ambient atmosphere and a total weight loss of similar to 26 wt % upon heating to 180 degrees C, above which the structure starts to decompose. The crystal structures of Cu-III[Fe-III(CN)(6)](2/3)center dot nH(2)O and K2/3Cu[Fe(CN)(6)](2/3)center dot nH(2)O were refined using synchrotron X-ray powder diffraction data. In both, one-third of the Fe(CN)(6) groups are vacant, and the octahedron around Cull is completed by water molecules. In the two structures, difference Fourier maps reveal three additional zeolitic water sites (8c, 32f, and 48g) in the center of the cavities formed by the-Cu-N-C-Fe- framework. The K-containing compound shows an increased electron density at two of these sites (32f and 48g), indicating them to be the preferred positions for the K+ ions.