Glass composite materials were investigated for use as a seal between Fe78Cr22 steel interconnects and cell components. Additions of MgO-filler to the sodium aluminosilicate (NAS) base glass increased the coefficient of thermal expansion of the composite to ∼12ppm and provided a sufficient match between the components to allow for thermal cycling of the interfaces. The optimisation of composite seals with long-term compliancy showed promising results in terms of being able to control both the bonding strength and interface reactivity between metal and glass. The reactivity increased with temperature and protection of the surface by pre-oxidation did not alter the picture. Etching the metal surfaces in combination with application of Mn-, Co-, or Ni-oxide coatings reduced the metal–glass interactions and for the Ni- and Co-oxide coatings facilitated an excellent bond and gradual changing interface between the glass and metal.Samples were tested in model experiments and performance tested in multi-cell stacks. Both pure NAS glass and NAS/MgO composite material developed gas tight sealing performance, when sealed at 950°C and showed low, constant levels of leakage throughout multiple thermal cyclings after more than 5000h run time in stack testing.