Abstract: 1-Hydroxyethylidenediphosphonic acid (HEDP, H_SL) has been used in zinc plating, but no information of its mechanism of deposition has been reported. The composition of the mixed complex in plating solution was determined by chemical analysis,IR spectra and conductometric titration.The results showed that the composition corresponded to K₆[Zn(HL) (CO₃)₂].Effects of the second ligand CO₃^2-on electrochemical behavior of plating solution has been investigated. The results indicate that the CO₃^2-has an important action on stability of the plating solution, dissolution of anode, polarization of cathode and throwing and covering power of solution.Effect of the form of mixed complexes on electrodeposition has been discussed. Restriction of application in determining electrodeposited complex ion on cathode by method of exchange current density has also been discussed.