The first neutral phenol subfraction of the aqueous extract of rosemary leaves was studied as corrosion inhibitor for the Al-2.5Mg alloy in a 3% NaCl solution at 25 degrees C. Experiments were carried out over a wide range of concentrations. The results show that the additive adsorbs on the alloy according to the Freundlich isotherm. The polarization curves show that the first neutral subfraction acts as a cathodic-type inhibitor. The impedance spectrum obtained was interpreted in terms of change in characteristics of the natural oxide film at the surface of the alloy. Equivalent circuits, which represent the system, are proposed and individual circuit elements are defined.