A novel methodology is presented for detecting the onset of debonding in clay-based nanocomposites. The procedure is based on constant illumination of the test specimen as it is subjected to tensile deformation by sequential strain-steps. After each strain-step, an image of the specimen is taken along its gauge length using a digital camera and the image is stored in a computer for later analysis using image analysis software. Test results from a nanocomposite containing a weak interface between the clay and the matrix indicate that interface debonding begins to occur above 1% strain, as evidenced by a reduction in the transmitted light through the specimen with increasing strain. Based on related research, the darkening of the specimen was interpreted as clay/matrix debonding. In contrast to the approximately 11% failure strain of the base epoxy resin, the nanocomposite specimen with the weak interface failed at 3.6% strain.