The effect of iodide-based complexing agent on the electrochemical performance of zinc-iodide flow battery was investigated. The addition of a complexing agent that will stabilize triiodide during charging can reduce the self-discharge and increase the charge and voltaic efficiency. The hydrogen evolution and kinetics of the both electrode reactions in the presence of the complexing agent was assessed. Cyclic voltammetry and Tafel analysis were performed to evaluate the influence of complexing agent on zinc electrochemical kinetics. The results indicated that the complexing agent enhances the cell performance by increasing the kinetics, which will enable operation of the battery at increased current density, or increases in the charge-discharge efficiency. At the positive electrode a deposit was observed on the electrode surface. The deposit was characterized by SEM and EDS and the deposition process was found to change significantly in the presence of the complexing agent. The charge-discharge characteristics of the electrolyte system were investigated in an H-type glass cell, and polarization curves were obtained for the two half-cell reactions.