The alkali metal and alkaline-earth metal uranyl iodates K-2[(UO2)(3)(IO3)(4)O-2] and Ba[(UO2)(2)(IO3)(2)O-2](H2O) have been prepared from the hydrothermal reactions of KCI or BaCl2 with UO3 and I2O5 at 425 and 180 degreesC, respectively. While K-2[(UO2)(3)(IO3)(4)O-2] can be synthesized under both mild and supercritical conditions, the yield increases from <5% to 73% as the temperature is raised from 180 to 425 degreesC. Ba[(UO2)(2)(IO3)(2)O-2]H2O), however, has only been isolated from reactions performed in the mild temperature regime. Thermal measurements (DSC) indicate that K-2[(UO2)(3)(IO3)(4)O-2] is more stable than Ba[(UO2)(2)(IO3)(2)O-2](H2O) and that both compounds decompose through thermal disproportionation at 579 and 575 degreesC, respectively. The difference in the thermal behavior of these compounds provides a basis for the divergence of their preparation temperatures. The structure of K-2[(UO2)(3-)(IO3)(4)O-2] is composed of (I)(infinity)(UO2)(3)(IO3)(4)O-2](2-) chains built from the edge-sharing UO7 pentagonal bipyramids and UO6 octahedra. Ba[(UO2)(2)(IO3)(2)O-2](H2O) consists of one-dimensional (1)(infinity)(UO2)(2)(IO3)(2)O-2](2-) ribbons formed from the edge sharing of distorted U07 pentagonal bipyramids. In both compounds the iodate groups occur in both bridging and monodentate binding modes and further serve to terminate the edges of the uranium oxide chains. The K+ or Ba2+ cations separate the chains or ribbons in these compounds forming bonds with terminal oxygen atoms from the iodate ligands. Crystallographic data: K-2[(UO2)(3)(IO3)(4)O-2], triclinic, space group P (1) over bar, a = 7.0372(5) Angstrom, b = 7.7727(5) Angstrom, c = 8.9851(6) Angstrom, alpha = 93.386(1)degrees, beta = 105.668(1)degrees, gamma = 91.339(1)degrees, Z = 1; Ba[(UO2)(2)(IO3)(2)O-2](H2O), monoclinic, space group P2(1)/c, a = 8.062(4) Angstrom, b = 6.940(3) Angstrom, c = 21.67(1), beta = 98.05(1)', Z = 4.