Abstract: In an ammoniacal citrate buffer medium of pH 10.2,a green colored coordination complex was formed by the reaction of Co(Ⅱ) ion with the chromogenic reagent of dibromo-p-methyl-dibromocarboxyazo,having its absorption maximum at the wavelength of 650 nm,and apparent molar absorptivity of 2.59×104L·mol-1·cm-1.Lambert-Beer′s law was obeyed in the range within 1.0 mg·L-1.The proposed method was used to determine cobalt in 2 samples of Zn-Co plating solutions,giving results in consistency with those found by the method given in literature,and average values of recovery and RSD (n=5) found were 100.5% and 1.65% respectively.