Investigations of changes in phase composition, mechanical properties, and microstructure of ZrO2-based plasma-sprayed thermal barrier coatings (TBCs) with 8 mol% CeO2, 19.5 mol% CeO2/1.5 mol% Y2O3, 35 mol% CeO2, and 4.5 mol% Y2O3 after long-term heat treatments at typical operation temperatures (1000 degrees -1400 degreesC) are presented. Experimental studies include X-ray diffractometry, mechanical testing, and scanning electron microscopy. Thermal cycling experiments also have been performed. TBCs with g mol % CeO2 contain mainly the tetragonal equilibrium phase and, therefore, show rapid failure because of the high amount of tetragonal --> monoclinic phase transformation, even after relatively short heat treatments (1250 degreesC/1 h), In the case of the other systems that consist mainly of the tetragonal or cubic nonequilibrium phases, TBCs with 19.5 mol% CeO2/1.5 mol% Y2O3 or 35 mol% CeO2 reveal a smaller amount of monoclinic phase after long-term heat treatments (1250 degreesC/1000 h) compared with TBCs containing 4.5 mol% Y2O3. TBCs containing 35 mol% CeO2 show a higher degree of sintering than the TBCs with 19.5 mol% CeO2/1.5 mol% Y2O3 and, therefore, a greater increase of the elastic modulus, Among the systems investigated, TBCs containing 4.5 mol% Y2O3 exhibit the highest resistance to failure in thermal-cycling experiments.