Extraction behavior of zinc and cadmium in the systems of bis(1,1,3, 3-tetramethylbutyl) phosphinic acid (HMBP)/cyclohexane-nitric acid and bis(1,1,3, 3-tetramethylbutyl) monothiophosphinic acid (HMTP)/cyclohexane-nitric acid was systematically investigated. The extraction mechanism was proposed through the log-log plot analysis, and the extracted species of zinc and cadmium might be Zn[H (MBP)(2)](2), Cd[H(MBP)(2)](2) in HMBP/C6H12-HNO3 system and Zn[H(MTP)(2)](2), Cd[H(MTP)(2)](2) in HMTP/C6H12-HNO3 system. Experimental results showed that the selectivity of HMBP to cadmium was remarkably improved by introducing the soft donor atom sulfur into HMBP molecule, and the quantitative mutual separation of cadmium from zinc could be achieved by a single extraction with HMTP as an extractant. This method may be expected to be useful for the determination of trace cadmium in high purity zinc
