Abstract: A method and apparatus for measuring the thickness of a thin coating, having a thickness on the order of 1 to 10 nm, of one material formed over a substrate of another material of significantly different atomic number, for example, a carbon coating on a ferromagnetic substrate. A primary radiation source, for example, of electrons or X-ray, creates low-energy secondary electrons in the substrate. The intensity of inelastically scattered electrons generally increases with film thickness. The secondary electron spectrum measured for a test sample is compared with the spectra for a plurality of similar reference samples of the same set of compositions, and a test thickness is thereby determined. The method may be practice on conventional electron spectrometers with the addition of some programmed analysis. Various techniques are available for extracting the data and comparing the test and reference data.