Potassium (K-6[Ge-6(mu-OO)(6)(mu-O)(6)(OH)(6)]center dot 4H(2)O, 1), cesium ammonium (Cs-4.2(NH4)(1.8)[Ge-6(mu-OO)(6)(mu-O)(6)(OH)(6)]center dot 8H(2)O, 2), and potassium ammonium (K-2.4(NH4)(3.6)[Ge-6(mu-OO)(6)(mu-O)(6)(OH)(6)]center dot 6H(2)O, 3) peroxogermanates were isolated from 3% hydrogen peroxide aqueous solutions of the corresponding hydroxogermanates and characterized by single crystal and powder X-ray diffraction studies and by Raman spectroscopy and thermal analysis. The crystal structure of all three compounds consists of cations of potassium and/or ammonium and cesium, water molecules, and centrosymmetric hexanuclear peroxogennanate anion [Ge-6(mu-OO)(6)(mu-O)(6)(OH)(6)](6-) with six mu-oxo- and six mu-peroxo groups. Peroxogermanates demonstrate relatively high thermal stability: the peroxide remains in the structure even after water release after heating to 100-120 degrees C. DFT calculations of the peroxogermanate [Ge-6(mu-OO)(6)(mu-O)(6)(OH)(6)](6-) anion confirm its higher thermodynamic stability compared to the hydroperoxo- and oxogermanate analogues.