Abstract: Studies have been carried out on the colour reactions of quaternary complexes of cobalt with chromazurol S(CAS), pyridine and its analogs and octadecatrimethyl ammonium bromide. The molar absorptivity of the quaternary complex is 1.05×10⁵1 mol^-1.cm^-1 at λ_max = 655nm. The composition and apparent stability constant of quaternary complex have been determined. Satisfactory results are obtained by the application of the quaternary complex in the spectrophotometric determination of small amounts of cobalt in the presence of large amounts of rare earth elements. The reaction mechanism has been discussed.