The first plutonium(III) borate, Pu-2[B12O18(OH)(4)Br-2(H2O)(3)]center dot 0.5H(2)O, has been prepared by reacting plutonium(III) with molten boric add under strictly anaerobic conditions. This compound contains a three-dimensional polyborate network with triangular holes that house the plutonium(III) sites. The plutonium sites in this compound are 9-and 10-coordinate and display atypical geometries.