The phase composition and sintering behavior of two commercially available ceramics produced by Daiichi Kigenso Kagaku Kogyo (DKKK) and Praxair have been studied. DKKK powders have been manufactured using a wet coprecipitation chemical route, and Praxair powders have been produced by spray pyrolysis. The morphology of the powders, as studied by scanning electron microscopy, has been very different. DKKK powders were presented as soft spherical agglomerates containing crystalline particles, whereas the Praxair powders were presented as sintered platelet agglomerates, up to long and thick, which consisted of smaller crystalline particles. X-ray diffraction analysis has shown that both DKKK and Praxair powders contained a mixture of cubic and rhombohedral phases: 79% cubic rhombohedral for DKKK powders and 88% cubic rhombohedral for Praxair powders. Higher quantities of the Si impurity level have been detected in Praxair powder as compared to DKKK powder by secondary ion mass spectroscopy. The morphological features, along with differences in composition and the impurity level of both powders, resulted in significantly different sintering behaviors. The DKKK powders showed a more active sintering behavior than of Praxair powders, reaching 93–95% of theoretical density when sintered at for . Comparatively, the Praxair powders required high sintering temperatures at . However, even at such high sintering temperatures, a significant amount of porosity was observed. Both DKKK and Praxair ceramics sintered at or above exist in a cubic phase at room temperature. However, if sintered at and , the DKKK ceramics exist in a rhombohedral phase at room temperature. The DKKK ceramics sintered at or above exhibit cubic to rhombohedral and back to cubic phase transitions upon heating at a temperature range, while Praxair ceramics exist in a pure cubic phase upon heating from room temperature to . However, if heated rather fast, the cubic to rhombohedral phase transformation could be avoided. Thus it is not expected that the observed phase transitions play a significant role in developing transformation stresses in electrolyte upon heating and cooling down from the operation temperatures.