Adsorption of a carboxylic acid model compound and of benzoic acid on iron from the liquid phase, in presence or not of an amine base, was studied using the rotating electrochemical quartz crystal microbalance (rEQCM). The investigated acid,. base couples are models for corrosion inhibitors in fully formulated protective coatings for mild steel. Adsorption resulted in a decrease of mass on the quartz electrodes because water is replaced in the volume occupied by the adsorbing organic molecule. A simple model taking into account the molecular volume of the adsorbent and of water molecules could explain the measured mass loss. The effect of the organic base was studied by characterizing the individual adsorption behavior of the acid and base parts. From the measured mass loss due to the water replacement upon adsorption, the adsorption coverage was calculated. The resulting isotherms followed a LangmuirFreundlich behavior from which the adsorption constant, the standard free energy of adsorption and the heterogeneity parameter could be calculated. A co-adsorption with the carboxylic acid was found in the presence of the organic base. Indeed, the organic base showed the highest adsorption constant on metallic iron. Both acid and base adsorbed more easily on metallic than on oxidized iron. A good correlation between the adsorption behavior from EQCM data and polarization curves was found.