Mechanical spectroscopy has been used to study the thermal stress relaxation at the interfaces in magnesium matrix composites. The mechanical loss spectra measured as a function of temperature show an intrinsic component superimposed on transient damping. A model of thermal stress relaxation by dislocation motion controlled by a solid friction mechanism, where the transient damping depends non linearly on (T) over dot/omega is developed. A systematic method is developed to identify the correct level of transient damping. (C) 2009 Elsevier B.V. All rights reserved.