The thermal decomposition of erbium oxalate hydrate (Er-2(C2O4)(3).6H(2)O) till 900 degrees C, in air and nitrogen, is investigated by nonisothermal gravimetry and differential thermal analyses. The activation energy (Delta E) is determined for each thermal decomposition step. The gaseous decomposition products are identified by infrared (IR)-spectroscopy. Intermediate and final solid products are characterized by IR-spectroscopy and X-ray diffraction (XRD). The results indicate that, in air atmosphere, following a stepwise dehydration at 90-370 degrees C, the anhydrous product was noncrystalline and thermally unstable. An oxycarbonate Er2O2CO3 is Obtained at 450 degrees C which decomposes near 600 degrees C into crystalline Er2O3- The atmosphere has no affect on the decomposition pathway. The oxide proportion and crystailinity improved on increasing the temperature to 800 degrees C. The volatile decomposition products were water vapor and carbon oxides.