Three high-purity SiAlON materials (Si6-zAlzOzN8-z, z = 1, 2, 3) were characterized with respect to both structure and viscous behavior of internal grain boundaries. Internal friction experiments provided a direct measure of the intrinsic viscosity of grain boundaries and concurrently revealed the occurrence of a grain-boundary interlocking mechanism that suppressed sliding. A residual glass phase (consisting of aluminum-rich SiO2) and nanometer-sized mullite residues were found at glassy triple-grain junctions of the z = 1 SiAlON, A low-melting intergranular phase dominated the high-temperature behavior of this material and caused grain-boundary sliding at temperatures as low as 1100 degrees C. A quantitative analysis of the grain-boundary internal friction peak as a function of oscillation frequency indicated an intergranular film viscosity of log eta approximate to 7.5 Pa.s at 1100 degrees C. Glass-free grain boundaries were a characteristic of SiAlON materials with z greater than or equal to 2, which yielded a significant improvement in refractoriness as compared to the z = 1 SiAlON material. In these materials, relaxation resulting from grain-boundary sliding was suppressed, and the internal friction curve simply experienced an exponential-like increase.