Deracemization of a racemic mixture of Delta- and Lambda-[Co(acac)(3)] (acac(-) = acetylacetonate) was successfully carried out with a chiral ruthenium(II) complex, Delta-[Ru(menbpy)(3)](2+) [menbpy = 4,4'-bis{(1R,2S,5R)-(-)-menthoxycarbonyl}-2,2'-bipyridine], in acetnitrile-H2O (7:3 v/v), in the presence of either triethylamine or NaOH under visible light irradiation (420 < lambda < 470 nm). The Lambda-isomer was obtained preferentially with the enantiomeric excess of 38%.