One failure mechanism of thermal barrier coatings composed of yttria-stabilized zirconia (YSZ) has been proposed to be caused, in part, by the transformation of the tetragonal phase of YSZ into its monoclinic phase. Normally, studies of phase evolution are performed by X-ray diffraction (XRD) and by evaluating the intensities of a few diffraction peaks for each phase. However, this method misses some important information that can be obtained with the Rietveld method. Using Rietveld's refinement of XRD patterns, we observed, upon annealing of YSZ coatings, an increase of cubic phase content, a reduction in as-deposited tetragonal phase content, and the appearance of a new tetragonal phase having a lower yttria content that coexists with the as-deposited tetragonal phase of YSZ.