In this paper, the stability constants of the o Phdn Co complexes and oxygenated complexes were determined by pH potentiometry for the first time. The determined method was verified with the known o Phdn Cu system by comprising the stability constants obtained from experiment results with those in literatures. The calculated results of oxygenated complexes were confirmed by spectrophotometry. All the results are agreed very well, which show the correctness of the determination. In this paper, the stability constants of the o Phdn Co complexes and oxygenated complexes were determined by pH potentiometry for the first time. The determined method was verified with the known o Phdn Cu system by including stability constants obtained from experiment results with those in literatures. The calculated results of oxygenated complexes were confirmed by spectrophotometry. All the results are agreed very well, which show the correctness of the determination.