Abstract: In the supporting electrolyte containing 0. 10M potassium hydrogen phthalate at pH4.0,a sensitive derivative peak of nickel(Ⅱ)-thiosemicarbazide complex was found by using single-sweep oscillopolarography. The peak potential is -0.80V (SCE). The change of the peak height with the concentration of Ni(Ⅱ) appeared linear in the range from 2.0×10^-7 to 8.0×10^-6M. The character of the peak wave has been studied. It is confirmed that the peak wave is a catalytic pre-wave with adsorptive property and the adsorbed species may be Ni(TSC)₂²⁺.