Although electron paramagnetic resonance (EPR) has long proven itself a valuable technique for the study of various organic free radicals1including those that are biochemically interesting, only recently has it come into use as a convenient and sensitive method for inorganic analysis. Guilbault and coworkers have demonstrated the usefulness of EPR methods for the determination of low concentrations of various ions of the first transition metal series both in aqueous solution2, 3, 4and extracted into organic solvents.5Moyer and McCarthy6have also demonstrated the application of EPR to quantitative inorganic analysis. Inasmuch as the area under the EPR first derivative curve is related to the number of unpaired electrons, this can serve as a measure of the concentration of a relatively large number of inorganic free radicals including most paramagnetic metal ions. Some of the Group VIII metal ions such as Ni(II), Pd(II), and Pt(II), while paramagnetic in their high spin complexes, however, do not exhibit an EPR signal at room temperatures because of their very rapid relaxation times.