Abstract: A sensitive polarographic adsorptive wave of the coordination complex formed between Co(Ⅱ) and 3-thiazolylazo-5-aminophenol (3-TAP) in a borate buffer of pH 8.0 was observed at the potential of -0.74 V (vs.SCE). Linear relationship was kept between the 2nd derivative of peak current (I″p) and concentration of cobalt(Ⅱ) in the range within 1.4 mg·L-1. It was proved that the polarographic wave was an adsorptive wave,and that the electrode reaction was an irreversible process,involving transfer of 2 electrons. Tests for interfering effects of co-exising ions to the values of I″p were also studied. The proposed method has been applied to the determination of cobalt in 3 samples of vitamin B injections giving results checked quite well with the labelled values,and values of RSD′s (n=6) found were less than 2.5%. Values of recovery found by standard addition method were in the range from 92.0% to 107.0%.