There is considerable interest in investigating new thermal barrier coating (TBC) materials that are able to withstand high-temperature environments, with a view to increasing the energy efficiency of gas turbines used in aerospace and power generation. A particular challenge is the degradation of the TBC due to infiltration of molten deposits known as calcium-magnesium-alumino-silicates (CMAS). CMAS originates from siliceous debris ingested with the intake air and limits the operating surface temperature of coated components. A primary mitigation mechanism for CMAS infiltration is the dissolution and recrystallization of the CMAS constituents into stable phases on the TBC surface. Yttria-stabilized zirconia and gadolinium zirconates are well known TBC materials, however a thermodynamic database that contains information on their reactivity and stability with CMAS has yet to be fully determined. In this project, the phase equilibria of the ZrO2-SiO2-GdO1.5 system, where silica is taken to be representative of the CMAS contribution, were studied. Oxide powders were synthesized via the reverse co-precipitation technique; compounds of varying stoichiometry were pressed into pellets and heat treated at 1400°C and 1600°C. Powder x-ray diffraction and scanning electron microscopy, coupled with energy dispersive x-ray spectroscopy, were used for phase identification and stoichiometry confirmation. Based on the results from these experiments, the 1400°C and 1600°C sections of the ZrO2-SiO2-GdO1.5 phase diagram were constructed.