ZrO₂-based ceramics doped with TaO_2.5 and YO_1.5 are of great interest for next-generation thermal barrier coatings (TBCs) used in gas turbine blades. Highly doped tetragonal compositions have shown a potential for phase stability at elevated temperatures (~1500 °C) that is coupled with outstanding fracture toughness. The design of advanced TBCs, however, requires comprehensive thermodynamic information on the ZrO₂-YO_1.5-TaO_2.5 system. Consequently, assessments of the ternary equilibria require an understanding of the constituent binary systems. While the ZrO₂-YO_1.5 and YO_1.5-TaO_2.5 binaries are well studied, data on ZrO₂-TaO_2.5 are incomplete. The aim of this project is to elucidate phase equilibria in the ZrO₂-TaO_2.5 system at temperatures relevant to TBC applications. Specimens were synthesized via reverse co-precipitation and heat treated at temperatures ranging from 1250 °C to 1500 °C for 100 h.  X-ray diffraction was used for phase identification, in addition to determining any relevant transformation temperatures in the terminal oxide phases (i.e. ZrO₂ and TaO_2.5) and the intermediate Ta₂Zr₆O₁₇ phase. Scanning electron microscopy was utilized to analyze specimen microstructure, with energy dispersive x-ray spectroscopy employed to determine the homogeneity ranges of ZrO₂, TaO_2.5 and Ta₂Zr₆O₁₇. Results from these experiments are then used to refine the ZrO₂-TaO_2.5 binary phase diagram between 1250 °C to 1500 °C. This in turn, provides a more complete evaluation of the ZrO₂-YO_1.5-TaO_2.5 system, which can promote the development of future TBC systems.