System and method for determining the composition of deposited thin films by acquiring multiple sequential X-ray spectra for a film of interest during the deposition process as the film thickness increases, computing intensities of peaks found in the X-ray spectra corresponding to elements present in the film material, followed by computing, for each pair of elements, ratios of corresponding peak intensities, graphing the intensities and ratios against a parameter correlated to the film thickness, and applying a physically meaningful function to the graphed data for best fitting the data down to a ratio RA/B(0) for each pair of the elements for a virtual film of zero thickness. Elemental concentrations ratio for each pair of elements is subsequently computed as a product of RA/B(0) and a factor which is specific for the pair of elements, constant for the instrument as set up, and independent of elements concentrations, and of film thickness.