The thermal behaviour of strontium hydroxide octahydrate was studied by thermal analysis, an X-ray diffraction technique and optical microscopy. Up to 210 ~ this compound transforms to the hexahydrate and then to the monohydrate. Anhydrous strontium hydroxide crystallizes in the tetragonal system and a polymorphous transformation may occur at 480 ~ . From 530 ~ on it decomposes in three stages. Amorphous strontium oxide is obtained at 700 ~ .When ceramic bodies containing free strontium oxide are hydrated, strontium hydroxide results, and this causes their deterioration.According to literature data two strontium hydroxide compounds exist in the hydrated state: the octahydrate and the monohydrate.Strontium hydroxide octahydrate -Sr(OH)~ 9 8 H20 -may be obtained by crystallization from an aqueous strontium hydroxide solution [1], or by precipitating strontium hydroxide from the nitrate with NaOH and washing the precipitate with cold ethyl alcohol [2]. It crystallizes in the tetragonal system with the unit cell constants a = 9 A and c = 11.58/k From a study of the solubility of St(OH)2 in water, Latimer [5] has shown that the octahydrate becomes metastable at a temperature of 80 ~ The existence of strontium hydroxide monohydrate -Sr(OH)2 9 H20 -is reported by Reinders and Klinkenberg [6], who consider that it is formed in solution at 87.7 ~ The monohydrate was also obtained by drying the octahydrate in vacuum or in a current of hydrogen at 45-50 ~ [7].