Bi₂O₃-nucleated melilite based glasses and glass-ceramics (GCs) in the system CaO–MgO–Al₂O₃–SiO₂–La₂O₃ have been appraised for solid electrolyte sealing applications in high-temperature electrochemical devices, such as solid oxide fuel cells and oxygen pumps . The structure of the glasses was assessed by Fourier transform infrared (FTIR) and ²⁹Si and ²⁷Al magic angle spinning nuclear magnetic resonance (MAS-NMR) spectroscopy. The crystallization kinetics and sintering behaviour were investigated by differential thermal analysis and hot stage microscopy. All glass compositions exhibited single-stage shrinkage behaviour. X-ray diffraction (XRD), in conjunction with the Rietveld-RIR (reference intensity ratio) technique, was employed to quantify the crystalline and amorphous phases in GCs sintered under non-isothermal conditions for 1 h within the operating temperature range (850–900 °C). Merwinite and melilite (the solid solutions of akermanite and gehlenite) were revealed as the major crystalline phases formed in GCs. The amount of GC in the melilite phase increased significantly from 850 to 900 °C at the expense of merwinite. The coefficients of thermal expansion (CTE (200–700 °C)), 10.2–10.9 × 10⁻⁶ K⁻¹ for the glasses and 9.8–11.2 × 10⁻⁶ K⁻¹ for the GCs, are in good agreement with those typical for solid oxide electrolytes, such as 8 mol% yttria-stabilized zirconia (8YSZ), and metallic interconnects, such as Sanergy HT. Long term thermal stability studies demonstrate stable behaviour for the crystalline phase assemblage and chemical composition, CTE (200–700 °C) values of 9.9–11.7 × 10⁻⁶ K⁻¹, and good adjoining performance with the metal and electrolyte. The well matched CTE values and good adhesion to the other components in both air and a reducing atmosphere allow us to propose further research into the parent compositions as stabilized zirconia sealants. Glasses demonstrating bulk nucleation may also attract interest for other functional applications in optical and electronic devices.