Quantitative interpretation of EPMA measurements was required for the construction of the Fe-Sn-C phase diagram. This involved the conversion of measured x-ray intensities into concentrations, which is difficult for a light element (such as C) being present among heavier elements (Fe and Sn). To meet this problem, firstly the various intensity-concentration conversion methods from the literature have been studied and are briefly surveyed. The Claisse-Quintin relation with constant influence coefficients was selected as the algorithm for multiple regression purposes, which was applied to reference standard data. Calibration functions are obtained for Sn and C, which are used in a simple iteration procedure. This procedure has been used for the conversion of Sn and C x-ray intensities into concentrations. The method shows a relative accuracy of about 5%.