The pseudo-binary In2O3-SnO2 phase diagram has been determined in the range of 1000-1650 degrees C using electron probe microanalysis (EPMA) and x-ray diffraction (XRD) analysis of solid-state sintered samples. The solubility of SnO2 in In2O3 was found to range from 1.3 mol% at 1000 degrees C to a maximum of 13.1 mol% at 1650 degrees C, indicating that commercial SnO2-doped In2O3 thin films are thermodynamically metastable. In2O3 was found to have negligible solubility in SnO2 throughout the temperatures examined. In this study two intermediate compounds, In4Sn3O12 and In2SnO5, were found. Each phase was found to be stable only at high temperatures, decomposing eutectoidally at 1325 and 1575 degrees C, respectively. This is believed to be the first report of the high temperature phase In2SnO5, which is attractive for future research as a transparent conducting oxide.