A glass from the system BaO-ZnO-NiO-SiO2 was used as a crystallizing glass seal for Crofer 22 APU steel. The glass was characterized with respect to crystallization, thermal expansion, and sealing behavior. In crystallized specimens, different barium silicates as well as a solid solution with the generalized compositions BaZn2-x Ni (x) Si2O7 were detected. The coefficient of thermal expansion can be adjusted between 9.3 and 12.5 x 10(-6) K-1, depending on the thermal treatment. An increasing crystallization temperature leads to an increase in the coefficient of thermal expansion. Sealing experiments were performed on Crofer 22 APU in inert atmosphere to prevent the joint from forming BaCrO4. Strong interfacial reactions occur which, depending on the processing temperature, lead to the formation of metallic nickel particles near the interface. The mechanism of this reaction is described and discussed. Sealing temperatures between 815 and 965 degrees C, lying in the range of the working temperature of high-temperature reactors, were applied.