In this work, three new glass‐ceramic compositions are designed and characterized as sealant materials for solid oxide electrolysis cells (SOEC), having operating temperature of 850°C. The crystallization and the sintering behavior of the glasses are investigated by using differential thermal analysis (DTA) and heating stage microscopy (HSM) respectively. The glasses show glass transition temperatures of 715‐740°C, while the coefficients of thermal expansion (CTE) of 9.3‐10.3 × 10⁻⁶ K⁻¹ (200‐500°C) are measured for the glass‐ceramics, matching with the CTEs of the other cell components. The compatibility between the glass‐ceramic sealants, the 3YSZ electrolyte and the Crofer22APU interconnect is examined by means of SEM and EDS, in the as‐joined condition and after 1000 hours at 850°C in air. Compositional changes in the glass‐ceramic sealants are reviewed and discussed with respect to the formed crystalline phases before and after the aging treatment at 850°C.