New lanthanide complexes associating two ligands, oxalate and oxydiacetate, have been prepared under an autogenous pressure at 200°C for 7 to 21 days by reacting an aqueous suspension of the corresponding lanthanide oxalate and oxydiacetic acid. These complexes are gathered into two families, [Ln(H 2O) x ] 2(C 2O 4)[O(CH 2CO 2) 2] 2 with x=3, Ln=Ce–Gd I and x=1, Ln=Ho–Lu including Y II, according to the number of water molecules. The structures have been solved for neodymium I and for yttrium II, respectively, from X-ray diffraction data on single crystal. Both families crystallize in the monoclinic symmetry but with different space groups, P21/c and C2/m for I and II, respectively. I has a double-chained structure in the form of ladder while II has a double-layered structure. The oxydiacetate ligands and the metal atoms build a chain or a layer. The oxalate ligands bridge two chains or two layers, respectively. There is no connection between these entities, double-chain or double-layer, but hydrogen bonds. While I has a compact structure, II presents a relatively open framework; about 12% of the unit cell is empty. In both families, the oxalate ligand is bischelating while the coordination mode of the oxydiacetate ligand changes from tridentate and monodentate in I to tridentate and bismonodentate in II. The thermal behavior of some compounds of these two families is presented.