Debonding processes in model composites under tensile deformation were investigated by acoustic emission analysis. The composites were prepared from epoxy and polyethylene matrix filled with glass beads of various sizes and with different coatings. The detected acoustic emission signals were identified as debonding processes at the filler-matrix interphase, and are discussed as a rupture process on the basis of the Weibull probability distribution function. For the model composites, the effect of the filler size is discussed using a theory based on Griffith’s criterion of rupture.