The mechanisms of phase destabilization upon aging of the metastable t phase of yttria-stabilized zirconia (YSZ) are poorly understood, despite its broad application in thermal barrier coatings. To provide insight, synchrotron X-ray diffraction (XRD) with a quadrupole lamp furnace is used to examine the temperature response, including thermal expansion and phase evolution, of a 9mol% (8wt%) YO(1.5)t-8YSZ. The thermal expansion of equilibrated YSZ powders ranging from 0 to 18.4mol% YO1.5 is also investigated to better understand the effect of composition on the thermal expansion anisotropy. The T-0(c/t) temperature for t-8YSZ is estimated to be 1640 degrees C. Full decomposition of the t phase into a coherent mixture of a Y-lean tetragonal phase (t) and a Y-rich cubic phase (c) that coarsen over time is observed at elevated temperatures; however, upon quenching, the t phase reappears in the diffraction profile. This supports our evolving understanding that the t phase observed by XRD in aged samples is a microstructural artifact due to the coherency strain between the t and c phases.