The reagent 1-(2-quinolylazo)-2,4,5-trihydroxybenzene (QAT6) has been used for developing a method for the spectrophotometric determination of cobalt (II). Various factors influencing the sensitivity of this method have been studied. These factors include wavelength, standing time and pH. The range of applicability of this method for the determination of cobalt has been determined. The sensitivity of the method has been compared with other spectrophotometric mthods used for the determination of cobalt. The effect of several ions as interferents with the determination of cobalt has been investigated. The Composition of the Co-QATB complex, its stability constant and its free energy of formation have been determined.