EDAX is commonly used to determine the chemical composition of thin films. We use this technique to know the stoichiometric ratio of FeS2 films. We have identified three main error sources, which can affect the obtained results and yield mistaken conclusions: (a) EDAX signals from the substrate components; (b) carbon thin film used to protect the reference sample against environmental corrosion; and (c) environmental oxidation of the thin film surface. In general, the contribution of these error sources decreases as the film thickness increases and it is strongly dependent on the incident electron beam energy. In this work, we present a methodology to reduce the influence of these error sources in the determination of the stoichiometric ratio of thin films by EDAX. In this manner, we have investigated the stoichiometric ratio of FeS2 thin films with different thicknesses. (C) 2003 Elsevier B.V. All rights reserved.