AbstractA method is described for the calibration of Ar Kα counting rates, measured using crystal spectrometers equipped with Ar‐flow proportional counters. Linear extrapolation from overvoltage corrected x‐ray yields of the elements with Kα x‐ray energies lower than those of the ArK absorption edge gives an accurate description of the experimental Ar Kα intensity ratios from counters operated at different values of gas pressure. Results of all‐element film analyses provided by a matrix correction procedure, ZAF, indicate an accuracy better than 5% relative for the determination of Ar concentrations ranging from 0.1