Deracemization of a racemic mixture of Î - and Î -[Co(acac)3] (acacâ =â acetylacetonate) took place with a chiral ruthenium(II) complex, Î -[Ru(menbpy)3]²⁺ (menbpyâ =â 4,4â ²-bis{(1R,2S,5R)-(â )-menthoxycarbonyl}-2,2â ²-bipyridine), under visible light (420â <â λâ <â 470 nm) irradiation in the presence of either triethylamine or NaOH. The enantiomeric excess (e.e.) of Î -[Co(acac)3] was 28% in acetonitrileâ water (7â â ¶â 3 v/v; ionic strength (Iâ )â =â 0.1 mol dm^{â 3} (KCl)) without acetylacetone and 38% with acetylacetone (50 mmol dm^{â 3}) added to the solution. Quenching experiments on Î -*[Ru(menbpy)3]²⁺ led to the conclusion that this deracemization occurs through combination of the preferential photo-reduction of Î -[Co(acac)3] by Î -*[Ru(menbpy)3]²⁺ to afford [Co(acac)2] and the thermal oxidation of [Co(acac)2] by Î -[Ru(menbpy)3]³⁺ to afford preferentially Î -[Co(acac)3]. The selectivity of deracemization remarkably depends on the solvent; the e.e. value is 51% in acetoneâ water (7â â ¶â 3 v/v), 38% in ethanolâ water (7â â ¶â 3 v/v), and 37% in DMFâ water (7â â ¶â 3 v/v). The e.e. value significantly decreases to 33% from 51% in acetoneâ water (7â â ¶â 3 v/v) when the KCl concentration is increased to 0.3 mol dm^{â 3} from 0.1 mol dm^{â 3}. When either KF or CH3CO2K is added to acetoneâ water (7â â ¶â 3 v/v) instead of KCl the deracemization proceeds much more rapidly without decrease of the e.e. value (â 50%). These counter anion effects are interpreted in terms that the reduction potential of [Co(acac)3] becomes more negative by addition of either F^â or CH3CO2^â.