Three new potassium uranyl borates, K-12[(UO2)(19)(UO4)(B2O5)(2)(BO3)(6)(BO2OH)O-10] center dot nH(2)O (TPKBUO-1), K-4[UO2)(5)(BO3)(2)O-4]center dot H2O (TPKBUO-2), and K-15[(UO2)(18)(BO3)(7)O-15] (TPKBUO-3), were synthesized under high-temperature/high-pressure conditions. In all three compounds, the U/B ratio exceeds 1. Boron exhibits BO3 coordination only, which is different from other uranyl borates prepared at room temperature or under mild hydrothermal conditions. A rare uranium(VI) tetraoxide core UO4O2, which is coordinated by two BO3 groups, is observed in the structure of TPKBUO-1. Both structures of TPKBUO-1 and TPKBUO-3 contain three different coordination environments of uranium, namely, UO4O2, UO2O4, and UO2O5 and UO2O4, UO2O5 and UO2O6 bipyramids in TPKBUO-1 and TPKBUO-3, respectively.