The metal content in Ziegler–Natta catalysts based on Ti and Mg derivatives could not be determined directly by x‐ray fluorescence (XRF) spectrometry owing to the instability of the catalyst. The samples were then calcined and the resultant Mg and Ti oxides were characterized by x‐ray diffraction (XRD). A series of synthetic standards were prepared with high‐purity MgO and TiO2, since there were no available certified standards for the catalysts. Samples and standards were compressed with high‐purity H3BO3 flux. This protocol was shown to be reproducible, having a correlation coefficient of 0.9999. The metal contents determined by XRF were in accordance with those obtained by a complexometric titration (Mg) and a UV–visible spectrophotometric method (Ti). Copyright © 2004 John Wiley & Sons, Ltd.