The grain-boundary structure of a model SiAlON polycrystal with nominal composition Si(5)AION(7) was characterized by transmission electron microscopy (TEM) both in an equilibrium (as-processed) state at room temperature and after quenching from elevated temperature. In addition, low-frequency (1-13 Hz) internal friction data were recorded as a function of temperature, showing a pronounced grain-boundary sliding peak positioned at 1030degreesC. High-resolution transmission electron microscopy (HRTEM) of the equilibrated low-temperature microstructure revealed residual glass only at multigrain junctions, but no amorphous intergranular films were observed. The detection of clean interfaces in the as-processed sample contradicts the internal friction data, which instead suggests the presence of a low-viscosity grain boundary phase, sliding at elevated temperatures. Therefore, a thin section of the as-sintered material was heated to 1380degreesC and rapidly quenched. HRTEM analysis of this sample showed, apart from residual glass pockets, wetted grain boundaries, which is in line with the internal friction experiment. This wetting-dewetting phenomenon observed in z = 1 SiAlON is expected to have a strong impact not only on high-temperature engineering ceramics but also on geological, temperature-activated processes such as volcanic eruptions.