Two complex layered uranyl borates, K-10[(UO2)(16)(B2O5)(2)(BO3)(6)O-8]center dot 7H(2)O (1) and K-13[(UO2)(19)(UO4)(B2O5)(2)(BO3)(6)(OH)(2)O-5]center dot H2O (2), were isolated from supercritical water reactions. Within these compounds, borate exists only as BO3 units and is found as either isolated BO3 triangles or B2O5 dimers, the latter being formed from corner sharing of two BO3 units. These anions, along with oxide and hydroxide, bridge between uranyl centers to create the complex layers in these compounds. U(VI) cations are found within uranyl, UO22+ units, that are bound by four or five oxygen atoms to create tetragonal and pentagonal bipyramidal environments. The most striking feature in this system is found in 2, where a [UO4(OH)(2)] unit exists that contains U(V) within a tetraoxo core with trans hydroxide anions; therefore, this compound is a mixed-valent U(VI)/U(V) borate. The presence of a 5f(1) uranium site within 2 leads to unusual optical properties.