A stable green solution of tricarbonatocobaltate(III) has been prepared and used for the redox titrimetric determination of plutonium in HNO3 medium. Quantitative oxidation could be achieved and excess oxidant could be destroyed by NaNO2. Pu(VI) was deter-ined by adding known excess of Fe(II) and carrying out potentiometric titration. The precision at the level of 0.5–5.0 mg was 2% RSD.