Statistics for Activities in the spinel solid solution Fe<inf>X</inf>Mg<inf>1-X</inf>Al<inf>2</inf>O<inf>4</inf> saturated with ?-Al<inf>2</inf>O<inf>3</inf> have been measured for the compositional range 0 < X < 1 between 1100 and 1350 K using a bielectrolyte solid-state galvanic cell, which may be represented as Pt, Fe + Fe<inf>X</inf>Mg<inf>1-X</inf>Al<inf>2</inf>O<inf>4</inf> + ?-Al<inf>2</inf>O<inf>3</inf>//(Y<inf>2</inf>O<inf>3</inf>)ThO <inf>2</inf>/(CaO)ZrO<inf>2</inf>//Fe + FeAl<inf>2</inf>O<inf>4</inf> + ?-Al<inf>2</inf>O<inf>3</inf>, Pt Activities of ferrous and magnesium aluminates exhibit small negative deviations from Raoult's law. The excess free energy of mixing of the solid solution is a symmetric function of composition and is independent of temperature: ?GE = -1990 X(1 - X) J/mol. Theoretical analysis of cation distribution in spinel solid solution also suggests mild negative deviations from ideality. The lattice parameter varies linearly with composition in samples quenched from 1300 K. Phase relations in the FeO-MgO-Al<inf>2</inf>O<inf>3</inf> system at 1300 K are deduced from the results of this study and auxiliary thermodynamic data from the literature. The calculation demonstrates the influence of intracrystalline ion exchange equilibrium between nonequivalent crystallographic sites in the spinel structure on intercrystalline ion exchange equilibrium between the monoxide and spinel solid solutions (tie-lines). The composition dependence of oxygen partial pressure at 1300 K is evaluated for three-phase equilibria involving the solid solutions Fe + Fe<inf>X</inf>Mg<inf>1-X</inf>Al<inf>2</inf>O<inf>4</inf> + ?-Al<inf>2</inf>O<inf>3</inf> and Fe + Fe<inf>y</inf>Mg<inf>1-Y</inf>O + Fe<inf>X</inf>Mg<inf>1-X</inf>Al<inf>2</inf>O<inf>4</inf>. Dependence of X, denoting the composition of the spinel solid solution, on parameter Y, characterizing the composition of the monoxide solid solution with rock salt structure, in phase fields involving the two solid solutions is elucidated. The tie-lines are slightly skewed toward the MgAl<inf>2</inf>O<inf>4</inf> corner.