This work is devoted to the corrosion inhibition of a carbon steel by a zinc salt/phosphonic acid association. Steady-state current-voltage curves and electrochemical impedance measurements carried out in the presence of each compound and for the mixture show a synergistic effect between the two molecules. The concentrations of the compounds in the mixture were lower than the concentrations used for each compound separately. Phosphonic acid was observed to act as an anodic inhibitor whereas cathodic action was shown for the zinc chloride. Electrochemical measurements and surface analysis (XPS and reflection-adsorption spectroscopy at grazing incidence) showed that the synergistic effect afforded by the mixture was attributable to the reaction of the phosphonic acid with the zinc salt. The inhibitor film acts as a protective layer impermeable to ionic or molecular diffusion. The film is very thin and homogeneous in composition. A chemical structure of the film is proposed.