The oxidation of cobalt(II) by [Co(ox)3]3-in 1,2-diaminoethane solutions proceeds by both inner-sphere and outer-sphere pathways leading to formation of [Co(en)2(ox)]+and [Co(en)3]3+with rate constants 3300 and 390 M-1s-1respectively, at 25.0 °C and 0.10 M ionic strength. When optically active [Co(ox)3]3-is used, both pathways show stereoselectivity. In the inner-sphere pathway, stereoselectivity is small, 1.5% ΔΔ, a consequence of the separation of the reacting centers by the oxalate bridge. In the outer-sphere pathway, stereoselectivity is 9% A. The effect on the stereoselectivity of varying the structure of the reductant for the series [Co(sep)]2+, [Co(sen)]2+, [Co(en)3]2+, [Co((±)-bn)3]2+, and [Co((±)-chxn)3]2+has been investigated for this pathway. Outer-sphere stereoselectivity has also been examined in the reactions of [Co(mal)3]3-with [Co(en)3]2+and [Co((±)-chxn)3]2+. The stereoselectivity data are consistent with a mechanism in which the carboxylate faces of [Co(ox)3]3- or [Co(mal)3]3-are presented to various orientations of the reductants in strong precursor ion pairs and are dependent on the relative importance of hydro-gen-bonding and electrostatic interactions between the complexes.