The mechanical behavior of multiphase materials is closely related to the interfacial adhesion between their various components. There is considerable interest in the development of simple experimental techniques for characterization of interfacial debonding during mechanical loading. Probably the best known method is tensile dilatometry, in which the onset and progression of debonding are related to the volume of microvoids generated in the material as it undergoes mechanical loading. Several authors have suggested that equivalent information can also be extracted from stressstrain data generated during a simple constant strain rate test. In practice, however, the transition between the initially well-bonded and the debonded state is obscured by the strain-induced softening of the matrix, which is usually observed in the same strain range as the debonding. In this work the filler/matrix debonding in polypropylene and polyamide 6 filled with up to 50 vol % of glass beads is examined using both tensile dilatometry and an analysis of tensile stress-strain curves. It was found that, depending on the level of adhesion, either a complete or partial debonding occurs in the strain range studied (0-8%). It appears that the volume change due to debonding is a small part of the total volume strain recorded. Therefore, the accuracy of the tensile dilatometry is not sufficient to detect the onset of debonding. The loss of stiffness of the composite, particularly when compared to the loss of stiffness of the matrix offers a more promising way to follow the debonding process.