A convenient approach is proposed for the quantitation of elemental cofactors in proteins and the determination of metal/protein stoichiometry, on the basis of energy dispersive X-ray fluorescence spectroscopy (EDXRF). The analysis of proteins containing the metals Cu, Fe, Zn, and Ca and also the nonmetallic element P is shown as a demonstration of the generality of the method. In general, the reported method gives limit of detection (LOD) and limit of quantification (LOQ) values in the low ppm range and requires only a few microliters of protein sample at micromolar concentrations. Moreover, sample preparation does not require any digestion steps before the analysis. The expected metal/protein stoichiometry was observed for each protein analyzed, highlighting the precision and accuracy of the method in all the tested cases. Furthermore, it is shown that the method is compatible with multimeric proteins and those with post-translational modifications such as glycosylation.