Solid solutions of zirconium dioxide and hafnium dioxide with magnesium oxide are unstable, and at 1200° gradually decompose into the original components. The solid solutions of zirconium dioxide and hafnium dioxide with calcium oxide also manifest a tendency to decompose at 1000–1200°, but the degree of decomposition is limited. The decomposition products in the system ZrO2-CaO and HfO2-CaO are monoclinic ZrO2 and HfO2, and CaZrO3 and CaHfO3, respectively. A substantial effect is exerted on the kinetics of the decomposition of solid solutions by impurities contained in the initial materials. The solid solutions ZrO2 with MgO obtained from industrial zirconium dioxide (98.3% ZrO2) at 1200° in 15–20 hours decompose almost entirely, whereas the degree of decomposition of the solid solutions obtained from zirconium dioxide (99.8% ZrO2) only attains 30% in 200 hours. The kinetics of the solutions HfO2 arid MgO are of the same nature under similar conditions. The degree of decomposition of the solid solutions of zirconium dioxide and hafnium dioxide with calcium oxide is also decreased to an appreciable extent when purer reagents are used. No substantial differences are noticed in the behavior of the solid solutions of zirconium dioxide and hafnium dioxide with magnesium and calcium oxides.