In a composite material, the degree of adhesion between the fiber and the matrix plays an important role in the overall performance of the material. Because the load between the fiber and the matrix is realized throughout the interphase region material, a lot of effort has gone into characterizing the strength of the interphase. In this study, nanoscratch tests on the composite samples were used to provide a relative measure of adhesion in different composite materials. Carbon-filled nylon 6,6 and polycarbonate resins were evaluated with this method. The carbon fillers we used were polyacrylonitrile-based carbon fibers sized and surface-treated for the respective matrix and pitch-based carbon fibers without any sizing or surface treatment. Tensile and X-ray photoelectron spectroscopy data for the composites we considered are also presented to compare to the nanoscratch results. It is shown that nanoscratch testing on the composites, with the proposed data analysis, can be an effective tool for determining the relative degree of adhesion between different composites. (C) 2006 Wiley Periodicals, Inc. J Appl Polyrn Sci 103: 328-335, 2007