Solid solution effects on thermal conductivity within the MgO-Al2O3-Ga2O3 system were studied. Samples with systematically varied additions of MgGa2O 4-MgAl2O4 were prepared and the laser flash technique was used to determine thermal diffusivity at temperatures between 200°C and 1300°C. Heat capacity as a function of temperature from room temperature to 800°C was also determined using differential scanning calorimetry (DSC). Solid solution in the MgAl2O4-MgGa2O4 system decreases the thermal conductivity up to 1000°C. At 200°C thermal conductivity decreased 24% with a 5 mol% addition of MgGa2O4 to the system. At 1000°C, the thermal conductivity decreased 13% with a 5 mol% addition. Steady-state calculations showed a 12.5% decrease in heat flux with 5 mol% MgGa2O4 considered across a 12 inch thickness.