Abstract: Using the electrode reaction of the ligand THAQ in Zr(Ⅳ)-THAQ complex, as we have reported previously, zirconium(Ⅳ) could be determined quantitatively by oscillopolarography in THAQ (when pretreated with NaNO₂)-borate buffer solution. In this paper a detail description of the properties of peak current of THAQ, the effect of NaNO₂ and other factors are presented. The number of electrons and hydrogen ions involved in the electrode reaction of THAQ were estimated. The third and fourth order of dissociation constants of THAQ were calculated to be k_a3=3.2×10^-10 and k_a4=2.0×10^-13, respectively. The electrode reaction mechanism of THAQ and Zr(Ⅳ)-THAQ complex for the given system has been proposed.