The thermal decomposition of the ammonium sul‐fate, carbonate, nitrate, oxalate, and sulfate salts of praseodymium, neodymium, and samarium was investigated thermogravimetrically in the temperature range 25° to 1400° C. Each salt was decomposed at a heating rate of 2.5° C per minute. The pyrolytic sequences have been identified, and the temperatures of formation of intermediates and of the sesquioxides have been determined. The sesquioxide formation temperatures increased in the order samarium, neodymium, and praseodymium irrespective of the anion. The nonstoichiometric ranges between Pr6O11and Pr2O3and the extent of these ranges have been determined for oxides derived from the carbonate, nitrate, and oxalate salts of praseodymium. The nonstoichiometric oxides of praseodymium were not observed in the decomposition of the alum and sulfate s