The objective of this study was to investigate the effect of heat treatments on the viscosities and electrical conductivities of glass sealants to be used in solid oxide fuel cells. Glass-based sealants, both with and without an alumina nanopowder added as a nanofiller, were heat treated at temperatures ranging from 750 degrees C to 770 degrees C for periods of up to 240 h. The effects of heat treatments on the viscosities, electrical conductivities and phase transformations of the sealants were investigated. The results showed that alumina nanopowder added to the glass increased both high-temperature electrical conductivities and the viscosities of the sintered glass nanocomposite sealants. However, lengthy heat treatments decreased the electrical conductivities of the glass nanocomposite sealants. This decrease in the conductivities of the heat-treated glass nanocomposites was attributed to the crystallization of glass phase, owing to the dissolution of the alumina nanofiller in the sealing glass.