Solid solution effects on thermal conductivity within the MgOAl2O3Ga2O3 system were studied. Samples with systematically varied additions of MgGa2O4MgAl2O4 were prepared and the laser flash technique was used to determine thermal diffusivity at temperatures between 200 degrees C and 1300 degrees C. Heat capacity as a function of temperature from room temperature to 800 degrees C was also determined using differential scanning calorimetry (DSC). Solid solution in the MgAl2O4MgGa2O4 system decreases the thermal conductivity up to 1000 degrees C. At 200 degrees C thermal conductivity decreased 24% with a 5mol% addition of MgGa2O4 to the system. At 1000 degrees C, the thermal conductivity decreased 13% with a 5mol% addition. Steady-state calculations showed a 12.5% decrease in heat flux with 5mol% MgGa2O4 considered across a 12inch thickness.