In this work, the 3,4-diaminobenzenesulfonic acid cobalt complexes were studied by the spectrophotometric method. The reaction rate is proportional with the increasing of extinction. Thus, Van’t Hoff’s differantial method can be applied, the initial reaction rates were found from the slopes at the initial points of the A560= f(Time) curves which shows the increasing of extiction with time. In this procedure, the reaction rates were determined and a relation was found between pH cobalt(II), ligand and oxygen concentrations and the reaction rate as given: Vo=∆A/∆t=K[OH-]2[]Co2+][L]2[O2]. By using the Job’s method of continuous variation, the composition of the complex was found as metal/ligand=1/2. The stability constant of complex was found as log K=5,93 and the activation energy of complex was calculated. The reaction rate increases proportionally with the light. The catalytic effect of the complex as an autoxidation catalyst was investigated. The complex formation has several steps as given below: At first, one Co2+ ion coordinate two ligand molecules and one oxygen molecule is bonded to the same Co2+ ion probably with the π bond. Then O2 removes from cobalt as a radical O2., and it oxidizes the Co2+ ion to Co3+ and takes two protons from the two amine groups of each ligand to form H2O2. Simultaneously, the somplex which has two oxidized monoimine groups coordinates two OH-. The end of the oxidation 3,4-diiminobenzenselfonic acid is formed. Keywords :