5 mol% yttria–zirconia ceramic was studied for its use as solid electrolyte in high-temperature pH sensors since this material has interesting mechanical and ion-conducting properties and is available on an industrial scale. However, although its fracture mode in air is transgranular, some batches showed severe intergranular cracking in the working medium, water at 300 °C. In a previous study, it was found that the degradation could be related to the stress caused by the martensitic transformation of intergranular tetragonal precipitates. The aim of present study was to prove this assumption by experiment. To simulate the effect of transforming grain boundary precipitates, a nearly precipitate free material was subjected to an externally applied stress while exposed to 300 °C water. It was found that this also led to degradation by intergranular fracture. This proved the effect of tensile stress, but it also showed that the occurrence of intergranular fracture cannot be attributed solely to the location of the transforming precipitates at the grain boundaries. In an attempt to find the cause of the stress-induced intergranular fracture, 8 mol% yttria–zirconia was tested as well but this material showed only transgranular fracture. As a possible explanation, it is suggested that the occurrence of intergranular fracture in the 5 mol% yttria ceramic is not a property of the pure yttria–zirconia system but that sintering additives or impurities, segregated at the grain boundaries, could be the cause. However, examination of such segregation was not foreseen in the present study. Nevertheless, based on the present results it is thought that it should be possible to reduce degradation by a strict decrease of the amount and size of the intergranular tetragonal precipitates.