Viscous energy dissipation in a liquid phase sintered silicon nitride, and the resulting internal friction, were investigated at temperatures up to 1300 degrees C with uniaxial tension-compression, torsion pendulum, and impulse excitation tests. The amount of internal friction, which is lower after heat treatment of the silicon nitride, reaches a steady-state value only after long-term tests at high temperature. A low and a high (> 50 MPa) stress amplitude regime are distinguished, revealing the existence of different anelastic deformation mechanisms. The evolution of the microstructure is investigated by X-ray diffraction, analysis of scanning electron microscopy images, and high resolution and analytical transmission electron microscopy in order to relate specific internal friction features to the silicon nitride microstructure. (C) 1998 Acta Metallurgica Inc. Published by Elsevier Science Ltd. Ali rights reserved.