The structural integrity of the sealing material is critical for the reliability of solid oxide fuel/electrolysis stacks. The current work concentrates on microstructural and mechanical aspects of a sealant material for this application. In particular, the crystallization behavior as a determining factor for the sealants’ mechanical behavior is investigated via high-temperature XRD for 24 h. Furthermore, regarding mechanical properties, three- and four-point bending tests are carried out on sealant bars and head-to-head joined specimens at room- and high-temperatures, yielding in particular relevant fracture stress data. In addition, the elastic modulus is measured by the impulse excitation test from RT to 900 ºC. Tests are done for both as-sintered (as-joined) and annealed samples. The main crystallization appears to happen during the initial joining time. The sealant shows a relatively stable flexural strength in terms of temperature dependency as well as effects of the aging process. In fact, the joined specimens reveal a more than 50% lower flexural strength than glass bars at all temperatures. A complementary finite element simulation indicates the presence of a non-negligible thermal residual stress in joined specimens.