After fabrication of an epoxy system filled with aluminum powder, followed by inelastic deformations under compression of the specimens, Positron Annihilation Lifetime Spectroscopy (PALS) was used to follow the evolution of the free-volume holes in the matrix. In order to describe the micromechanical deformation mechanism that takes place in the matrix around the inclusions, the experimental free-volume holes data were analyzed in terms of a model specifically developed. This model involves a hydrostatic internal stress resulting from the fabrication process of the composites and the deviatoric part of the applied stress during inelastic deformation. The influence of both kinds of stresses on the modification of the free-volume sizes in the matrix is discussed. (c) 2005 Elsevier Ltd. All rights reserved.