Magnesium uranate was prepared by standard solid‐state synthetic techniques. Decomposition pressures were measured between 1000° and 137OOC. At oxygen contents less than MgUO3.5a solid solution of fluorite structure was found. Above MgUO3.75the orthorhombic magnesium uranate phase was observed. The two phases were in equilibrium in air at 126OOC. The heat of solution of O2in the fluorite phase was 45 kcal mole−1and the heat of the reaction MgUO3.5+ 0.12O2→ MgUO3.75was 83.3 kcal mole