Standard applications of quantitative XRF are determinations of composition of bulk specimens, composition and thickness of thin multielement layers and thickness of thin element layers. The theoretical background was given by Sherman, and Shiraiwa and Fujino Numerous approaches like empirical coefficients and fundamental parameters were developed and deliver valuable results. Extending the methods to a quantification of very thin layers consisting of more than one chemical element, it becomes more and more complicated to split the solutions into thickness and composition. The explanation is given by the improving validity of the linear approach for the measured signals in dependence on thickness and composition due to the thin layer approximation exp(x)≈l+x. This leads finally to the finding that measured characteristic signals are proportional to the product c-t, where c represents the concentration of the chemical element under investigation and t the thickness of the layer.