We present the synthesis of new biuret complexes NH2CONHCONH2 (BU), PrBr3·2BU·4H2O (I) and HoBr3·2BU·4H2O (II), and the results of X-ray diffraction analysis. In the crystals of I and II we find complex cations [Ln(H2O)4(BU)4]3+ (Ln = La, Gd) and uncoordinated halogenide ions existing in the external sphere. Biuret molecules serve as bidentate ligands (through the oxygen atoms) and are transformed from the original trans- to cis-configuration. The coordination polyhedron is a distorted square antiprism. These compounds have one or two non-coordinated molecules of water and one non-coordinated BU molecule (II) existing in the external sphere, which are joined with the halogenide ions by hydrogen bonds
