Transmission electron microscopy (TEM) is used to study the effect of high-temperature tests on the amorphous grain boundary films in a commercial, liquid phase sintered silicon nitride ceramic. The thickness of amorphous films between silicon nitride grains is characterized by high-resolution TEM. In addition, electron energy-loss spectroscopy is applied to analyze the composition of grain boundary films. A smaller equilibrium film thickness is measured in the specimens after testing. This decrease in film thickness is due to changes in the intergranular film chemistry. The composition changes are caused by phase transformations in the secondary crystalline phases, which were observed by TEM and X-ray diffraction, and by crystallization of residual amorphous pockets during high-temperature testing. (C) 1998 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved.