AbstractA thermal analysis study of the one‐stage decomposition of small (mg) quantities of carnallite, KCl. MgCl2, 6H2O → KCl + MgO + 5H2O ↑ + 2HCl ↓, showed that dehydration and hydrolysis require temperatures above 300 °C. Decomposition of greater amounts, and analysis of the products, showed that at 450 °C, 88 to 90% of the magnesium occurs as chloride‐free high‐purity periclase. At higher temperatures increasing amounts of chlorides remain in the periclase and are difficult to separate. Unglazed porcelain is a suitable material for the decompo