The Na-Cs and Na-Sr exchange properties of Na-montmorillonite heated at various temperatures under dry conditions have been studied at 25°C and a constant normality of solution. The standard free energy changes were-2.6 kcal/eq for Na-Cs and-0.99 kcal/eq for Na-Sr in the air-dried montmorillonite. In the montmorillonite pre-heated at 100°C-600°C, the Cs-selectivity increased in the smaller Cs-equivalent fraction in clay but such an increase did not appear in the Na-Sr exchange. In addition, complete exchange from Na-form to Cs-and Sr-forms was not achieved in the pre-heated materials because of fixation of interlayer Na. The proportions of fixed Na were less than 10% upon heating at <300°C but increased to 25-30% for Na-Cs and 15-20% for Na-Sr by heating at 600°C. These values were nearly constant indifferent to normality of solution and exchange temperature.