Periodate is a strong oxidant and is often reduced to IO3- or I-2 under hydrothermal conditions. Here, we present a rare case of a mixed-valent iodate(V)/periodate (VII) compound, Th(H2O)((IO3)-O-v)(2)[I-VII (0.6) V1.76O7(OH)], prepared with a hydrothermal method starting from periodic acid. Crystallographic results demonstrate that heptavalent iodine adopts (IO6)-O-VII distorted octahedral geometries, which are stabilized on the crystallographically compatible crystal lattice sites of VO6 octahedra through an aliovalent substitutional disorder mechanism. X-ray photoelectron and synchrotron radiation X-ray absorption spectroscopes both quantitatively confirm the presence of mixed valent iodine oxo anions with a molar ratio (I-V/I-VII) of 4:1, consistent with the single crystal X-ray analysis. The crystallization of mixed-valent products with compatible lattice site can be fancily utilized for stabilizing the uncommon oxidation states of other elements in general.