Sealant materials for solid oxide fuel cells (SOFCs) must meet a demanding set of performance criteria for operating lifetimes of up to 40,000 hr. The resulting seals must be gas tight at temperatures up to 1000°C, resist stresses from thermal gradients and expansion mismatch of different stack materials, and perform reliably over long times at high temperatures in both oxidizing and reducing atmospheres. Ceramic and metal filled glass composite sealants provide for greater design flexibility than other approaches. The seal properties can be tailored by varying the composition, amount, and microstructure of the particulate phase. Composite properties such as glass transition temperature, viscosity, and thermal expansion coefficient can be altered by rational control of the glass chemistry and composite microstructure. Several specific materials combinations have been engineered to meet the demanding set of criteria for sealing materials in SOFCs and characterized by means of viscosity measurements at the proposed operating temperature of 750°C The influence of the matrix/particle interactions has been separated from the mechanical effects of the added phase in studies that systematically vary the chemical composition of the particles. Models for composite suspension viscosity were also used to interpret the observed variation in viscosity with composition and volume fraction of the filler.