Oxidation of thiocyanate by tris(1,10-phenanthroline)Fe(III) to give cyanide and sulfate ions was studied at 25 °C in both H2SO4 and HClO4 media. The rate of oxidation showed a first order dependence on the Fe(III) complex and a second order dependence on potassium thiocyanate at ≥ 4.0 × 10−4 M potassium thiocyanate. The value of kobs decreased with increasing [H+]. This acid inhibition is consistent with the protonation of SCN− to give an inactive HSCN species. A mechanism consistent with the data involved an initial equilibrium formation of [Fephen3SCN]2+ which subsequently undergoes an outer-sphere electron transfer with SCN− to eventually give Fe(II)phen3, CN−, and SO42−. Similar kinetic behavior was found with the substituted tris(1,10-phenanthroline)Fe(III) complexes. The value of the third order rate constant was found to increase with the increase in the reduction potential of the complexes. This result indicates a direct dependence of the standard free energy of the reaction and the free energy of activation.