Abstract The crystal structure of La(H 2O) 2K(C 2O 4) 2·H 2O has been solved ab initio from powder diffraction data collected with conventional monochromatic X-rays. The symmetry is monoclinic, space group C2/ m, cell dimensions a=22.033(1) Å, b=7.6003(5) Å, c=6.6418(4) Å, β=103.814(6)°, V=1080.05(8) Å 3, Z=4. Since a disorder of one oxalate group is observed with all possible monoclinic space groups, it has been found convenient to describe the crystal structure in a triclinic subcell. The disorder problem is thoroughly discussed with respect to the powder diffraction data available for this material. From this description the compound exhibits a layered-type structure, which could explain reported cation-exchange properties. The layers are formed by four-membered rings [La(C 2O 4)] 4. The potassium atom and two of the three water molecules are located between the layers. The third water molecule lies on the layer in tunnels with a diamond-shaped cross section. The lanthanum atom is tenfold-coordinated in a distorted bicapped square antiprism and the potassium atom is eightfold-coordinated in a dodecahedron. The crystal structure of La(H 2O) 2(NH 4)(C 2O 4) 2·H 2O is isostructural with that of La(H 2O) 2K(C 2O 4) 2·H 2O [ a=22.130(4) Å, b=7.774(1) Å, c=6.655(2) Å, β=105.28(2)°, V=1104.4(3) Å 3]. The thermal decomposition of these two precursors has been carried out by means of temperature-dependent X-ray diffraction and TG-DSC. The dehydration leads to anhydrous phases, which are amorphous and poorly crystalline for the potassium and ammonium compounds, respectively. Amorphous LaK(C 2O 4) 2 crystallizes at 260°C. Its symmetry is monoclinic with the cell dimensions a=5.687(3) Å, b=15.241(5) Å, c=9.017(4) Å, β=92.82(3)°. At higher temperature, the decomposition of LaK(C 2O 4) 2 yields La 2O 2CO 3 (Type-Ia), La 2O 2CO 3 (Type-II), and pure La 2O 3, successively. La(NH 4)(C 2O 4) 2 leads to La 2O 2CO 3 (Type-II) and, finally, La 2O 3.