The inhibition behaviors of 5,6-diamino-1,10-phenanthroline (DAPTL) and chloride ion on the corrosion of mild steel in 5.5 M H3PO4-2% H2SO4 solution have been studied by weight loss, potentiodynamic polarization and electrochemical impedance spectroscopy techniques. The results indicate that combination of DAPTL and Clshows stronger synergistic inhibition effect on mild steel corrosion in 5.5 M H3PO4-2% H2SO4 solution. The Electrochemical studies indicate that DAPTL alone or the combination of DAPTL and Clacts as a mixed type inhibitor retarding the anodic and cathodic corrosion reactions and does not change the mechanism of either mild steel dissolution or hydrogen evolution reaction. The corrosion reaction is controlled by charge transfer process. The adsorption of the inhibitors on the mild steel surface follows the Langmuir adsorption isotherm model. The thermodynamic and kinetic parameters (Kads, ΔG o ads, ΔH o ads,ΔS o ads, and Ea) have been calculated and discussed. The adsorption of inhibitor molecules on the steel surface is a spontaneous process with physical and chemical adsorption, while chemical adsorption is dominant.