Abstract: In general, the cobalt (Ⅱ) complex is oxidated quite easy in the bis(2-ethyl-hexyl)dithiophosphoric acid solvent extraction system by oxygen in air. The oxidizing process of cobalt( Ⅱ ) has been studied in the bis(2-ethylhexyl)dithiophosphoric acid-pyridine solvent extraction system by using the methods of the interfacial tension, visible spectrum and infrared spectrum. The experiments have demonstrated that pyridine and its some derivatives can restrain the oxidation of cobalt( Ⅱ ) complex because of forming the mixed type complexes when they are coordinated into the Co( Ⅱ ) complex and the restraint ability of oxidation depends on the stability of mixed type complex formed.