Abstract: High sensitive color reaction of a new reagent of 2-(5-iodine-2-pyridylazo)-5-dimethylaminoaniline (5-I-PADMA) with cobalt (Ⅱ) has been studied. In pH 4.2~9.2 medium, 5-I-PADMA reacts with cobalt (Ⅱ) to form a violet-red complex exhibiting two absorption peaks at 554 nm and 602 nm respectively. This complex can be changed into another stable blue-green diproton species after acidification with mineral acids, and the two absorption peaks shift to 568 nm and 614 nm respectively. This species possesses a higher absorption characteristic. The optimal acidification conditions were found to be 0.3~7.0 mol/L HCl, 0.3~6.0 mol/L HClO₄, 0.3~4.2 mol/L H₂SO₄ and 0.7~7.3 mol/L H₃PO₄, respectively. The apparent molar absorptivity ε_{614 nm} is 1.21×10⁵ L /( mol· cm). Beer's law is obeyed in the range of 0~0.5 μg/mL for Co. The method provides high sensitivity, high selectivity and less interference. The method has been successfully applied to the determination of cobalt in ores with satisfactory results.