The synthesis and X-ray structure of the novel polymeric lanthanum(III) oxydiacetate complex [La-2-(C4H4O5)(3)(H2O)(3) . 5H(2)O](n) (1) is reported. One metal ion is coordinated to three tridentate oxydiacetate ligands, the other, to six outer carboxylate oxygens and three oxygens from metal-bound water molecules. The crystal structure of 1 shows it to be a three-dimensional polymer where the lanthanum centers are bridged by the carboxylate groups of the oxydiacetate ligands. Upon thermolysis 1 is converted into hexagonal La2O3.