The thermal decomposition of calcium oxalate doped with lanthanum, samarium or gadolinium has been investigated using thermogravimetry (TG) and differential thermal analysis (DTA). The kinetics of the decomposition steps have been studied by the non-isothermal TG technique. The doped oxalates decompose in a similar way to pure CaOx. After dehydration, decomposition of doped oxalates proceeds in two overlapping exothermic stages, i.e. decomposition of lanthanide oxalates followed by that of calcium oxalate. Samples heated up to 1000 degrees C reveal the existence of CaO and Ln(2)O(3) in separate phases.