The rate equation –d[FeII]/dt=kobs[FeII][MnIII] was established by measurements of the iron(II) diffusion current. For 0.3–15°C in 0.54-3 M HClO4 the variation of kobs was consistent with the relation kobs=(ko[H+]+k1Kh)/([H+]+Kh), where Kh is the manganese(III) hydrolysis constant. Values of k0 are just less than k1 as in the comparable oxidation of vanadium(IV). It is concluded from further comparisons of rates that the activated complexes are outer-sphere.