The occurrence of monoclinic zirconia phase has an important impact on the performance of thermal barrier coatings (TBC) of yttria-stabilized zirconia (YSZ). Therefore, a reliable method is needed to detect its contents and to investigate also its spatial distribution within the parent microstructure. This was the motivation to apply cathodoluminescence (CL) spectroscopy. YSZ coatings with different porosities were manufactured by atmospheric plasma spraying. CL analysis yielded monoclinic phase contents of 5.2 ± 1.6% for the high-porous sample and 3.4 ± 0.5% for the low-porous sample. The results were qualitatively confirmed by x-ray diffraction (XRD). However, due to its lower detection sensitivity the XRD results are quantitatively on lower level. Owing to its synthesis method, the applied powder feedstock showed a considerable content of monoclinic phase. The lower the particle temperatures were the larger fraction of monoclinic phase remained untransformed. This has to be considered when spraying high-porous TBCs.