The present study compares the feasibility of controlling the properties of a glass composite seal by adding nano- or micronscale yttria-stabilized zirconia (YSZ) powders to a borate glass used for sealing electrolyte-supported solid oxide fuel cells (SOFCs). The crystallization of the glass composites was found to be independent of the volume fraction of added YSZ, for both sizes of the additive. The variation of the flow properties of both composite seals was measured using a wettability test, and an increase of the contact angle was observed when the volume fraction of additives was increased. The major factor found to decrease spreading of the glass composite was the additive particle size, where shape retention was observed for the nanometer (nm)-YSZ composites while spreading of the micrometer (pm)YSZ composites was observed under the same testing conditions. Examination of the microstructure showed that initially the Ba-containing glass reacted with YSZ to form a BaZrO3 compound. Long-time exposure at 800 degrees C caused a large reduction in the coefficient of thermal expansion (CTE), which can be explained by increased formation of BaZrO3 and further change in glass composition. This change in CTE occurs rapidly for the nm-YSZ composites, which is not observed for the mu m-YSZ composites. However, the adverse reactions occurring between the additives and the glass matrix were found to reduce the CTE of the glass composites to a value lower than the recommended limit for a system used for sealing SOFCs.