Abstract: An X-ray fluorescent method for performing measurements on a thin layer provided on a substrate, notably a silicon substrate provided with a layer of tungsten silicide (WSix). The layer thickness and the concentration of the chemical elements in the layer can be determined in known manner by measurement of the intensity of a hard and a soft X-ray line in the fluorescent radiation. In conformity with the invention, the reliability and/or the accuracy of this determination can be established by measurement of the intensity of a third X-ray line, preferably being a line whose wavelength lies between that of the first two lines. Using a minimization procedure (least squares method applied to &khgr;2), the optimum values of c and d are determined. The accuracy of the determination can be established by variation of the optimum value of &khgr;2.