Phase evolution accompanying the isothermal aging of free-standing air-plasma sprayed (APS) 7–8 wt% yttria-stabilized zirconia (8YSZ) thermal barrier coatings (TBCs) is described. Aging was carried out at temperatures ranging from 982°C to 1482°C in air. The high-temperature kinetics of the phase evolution from the metastable t′ phase into a mixture of transformable Y-rich (cubic) and Y-lean (tetragonal) phases are documented through ambient temperature X-ray diffraction (XRD) characterization. A Hollomon–Jaffe parameter (HJP), T[27 + ln(t)], was used to satisfactorily normalize the extent of phase decomposition over the full range of times and temperatures. Comparison to vapor deposited TBCs reveal potential differences in the destabilization mechanism in APS coatings. Furthermore, the lattice parameters extracted from Rietveld refinement of the XRD patterns were used to deduce the stabilizer concentrations of the respective phases, which suggest a retrograde tetragonal solvus over the temperature range studied. In concert with a complementary microstructural study presented in Part II, this effort offers new insights into the mechanisms governing the phase evolution and raises implications for the high-temperature use of 8YSZ ceramics.