In a method for the determination of the measuring uncertainty for a device for X-ray fluorescence slice thickness measurements during measurement of a layer of a sample under investigation, a spectrum S(K) simulating an actual fluorescent radiation spectrum is generated for the channels K of a spectrum and for a given thickness d of the layer and, in each channel K, a random number generator is used to repetitively accumulate a random value to construct a total number N(K) of events registered in the K-th channel in measurement time t and the standard deviation .sigma.(d) is determined from the spectrum of slice thicknesses d extracted by means of the repetitive random values as measure of the measurement uncertainty.