The thermal stability of invert glass-ceramics, based on crystalline pyro- and orthosilicate phases, developed for hermetic seals for solid oxide fuel cells (SOFC) are described. The effects of long-term (up to 60 days) high temperature (up to 800°C) heat treatments on the properties of the glass-ceramics in oxidizing and reducing environments were evaluated by dilatometric analyses, x-ray diffraction, weight loss measurements, and impedance spectroscopy. The glass-ceramics have thermal expansion coefficients in the range 9.5-11.5 × 10-6/°C and can be sealed to SOFC materials, including Y2O3-stabilized ZrO2 (YSZ) and Cr-steel interconnect alloys, at or below 900°C. The thermal expansion characteristics of some glassceramics remain essentially unchanged after >40 days at 800°C, and the glass conductivity at 750°C remains constant (in the range 0.3-1.0 × 10-7 S/cm) in forming gas.