Equilibrium phase relationships in the MgIn2O4–MgGa2O4system at 1400°C were determined by X-ray diffraction. Single phases of the inverse-spinel-type MgIn2O4, the layered MgInGaO4, and the inverse-spinel-type MgGa2O4were observed in a compositional range of 0≤x≤0.35, 0.8≤x≤1.0, and 1.5≤x≤2.0 in MgIn2−xGaxO4, respectively. Rietveld structural refinement revealed that the layered MgInGaO4(space groupR3m,a=3.3060(4),c=25.832(3) Å) comprised a InO−2single octahedral layer and a MgGaO+2double trigonal-bipyramid layer alternatingly stacked along thez-axis. With increasing gallium content, room-temperature conductivity decreased, whereas transparency and optical band gap increased. Results of single-point energy calculation using the CASTEP code supported that MgIn2O4is a good conductor with a band gap of ∼3.5 eV and that MgGa2O4is a poor conductor with a band gap of ∼5 eV.Copyrigh 1999 Academic Press.