For the development and metrological certification of the procedure for measuring the mass fractions of rhenium in metallurgical samples by the extraction-X-ray fluorescence method, solid-phase samples-emitters were prepared to construct a calibration characteristic. The article evaluates the values of the expanded uncertainty of the emitter samples using the Bayesian approach. The sources of uncertainties associated with the preparation of the standard solution, the preparation of the extractant, the extraction process and the measurement of the analytical signal intensity were identified: the error of a volumetric flask and a pipette; the error in measuring the time and temperature of extraction, the error in the expansion of the volume of the flask at the stage of preparation of solutions, the error in the scales when measuring the mass of the extractant and when measuring the weight of the weighed portion of ammonium perrhenate, the error in measuring the optical density on an X-ray fluorescence spectrometer for the prepared emitter samples, repeatability of the results when measuring the analytical signal - emitters, linearity of the calibration characteristics. A quantitative assessment of each source of uncertainty was made, the contribution to the value of expanded uncertainty of all stages of the process was estimated. It was found that the greatest contribution to the value of the total uncertainty is made by the error associated with the error at the stage of preparation of the extractant and the stage of extraction concentration and extraction of the analyte from the liquid phase to the solid. An equation for calculating the total standard uncertainty is derived. The results obtained can be used in the metrological substantiation of the procedure for performing measurements of the ex-traction-X-ray fluorescence determination of the rhenium content in metallurgical samples.