Reactions of LnBr(3) or LnOI with molten boric acid result in formation of Ln[B5O8(OH)(H2O)(2)Br] (Ln = La-Pr), Nd-4[B18O25(OH)(13)Br-3], or Ln[B(5)0(8)(OH)(H2O)(2)I] (Ln = La-Nd). Reaction of PuOI with molten boric acid yields Pu[B7O11(OH)(H2O)(2)I]. The Ln(III) and Pu(III) centers in these compounds are found as nine-coordinate hula-hoop or 10-coordinate capped triangular cupola geometries where there are six approximately coplanar oxygen donors provided by triangular holes in the polyborate sheets. The borate sheets are connected into three-dimensional networks by additional BO3 triangles and/or BO4 tetrahedra that are roughly perpendicular to the layers. The room-temperature absorption spectrum of single crystals of Pu[B7O11(OH)(H2O)(2)I] shows characteristic f-f transitions for Pu(III) that are essentially indistinguishable from Pu(III) in other compounds with alternative ligands and different coordination environments.