The addition of small amounts (2-10 wt %) of SiO2 to gamma-Al2O3 increases the temperature of heat treatment necessary for transformation to alpha-Al2O3 by similar to 100 K. We have studied this system using high-temperature solution calorimetry in molten 2PbO . B2O3 at 1043 K, Our results indicate that the spinel-type Al2O3-SiO2 solid solutions with 2-10 wt % SiO2 are always energetically metastable by 30-35 kJ.mol(-1) (on a 4 O2- per mole basis) with respect to alpha-Al2O3 and quartz. Calculation of the maximum configurational entropy of the solid solutions allowed determination of the likely most negative value of the Gibbs free energy of the materials, The solid solutions are somewhat entropy stabilized, but still thermodynamically metastable by > 10 kJ.mol(-1) at 1400 K, Therefore, SiO2 addition appears to provide mainly a kinetic hindrance to alpha-Al2O3 formation.