2′-Dipyridyl-2,4-dinitrobenzoic acid-organic molecular complex (DDC) has been found to give a red-violet indicator reaction with cobalt(II) in pH range 10-14. Colour solutions of metal: ligand complex (1:1) absorb maximally at λ max 540 nm (ε_max =6.8·10³ L·mol^-1·cm^-1), the absorbance maximum of the reagent (DDC) alone being at 240 nm. This colour reaction has been used for determination of cobalt in aqueous solutions. The reagent is highly sensitive and selective for cobalt. The calibration graph is liniar over the range 0-12 μg·mL^-1 of cobalt per 5 mL of DDC solution (0.01 mol·L^-1).