Magnesium uranate was prepared by standard solid-state synthetic techniques. Decomposition pressures were measured between 1000° and 137OOC. At oxygen contents less than MgU03.5 a solid solution of fluorite structure was found.Above MgU03.7s the orthorhombic magnesium uranate phase was observed. The two phases were in equilibrium in air at 126OOC. The heat of solution of O2 in the fluorite phase was 45 kcal mole-1 and the heat of the reaction MgU03.6 + 0.1202 -+ MgU03.75 was 83.3 kcal mole-' ( 0 2 ) .