The effect of hexadecyl pyridinium bromide (HPB) and hexaclecyl trimethyl ammonium bromide (HTAB) on the corrosion behaviour of iron and copper in hydrochloric and sulphuric acid solutions has been investigated by potentiodynamic polarization and Tafel extrapolation methods. The polarization curves indicate that the two compounds behave as mixed inhibitors, but the cathode is more inhibited. HPB is more effective than HTAB in both acids; this is explained on the basis of the charge located on the nitrogen atom on the two compounds. The inhibition efficiency of the compounds investigated is more effective for iron and copper metals in HCl than in H2SO4, which is explained on the basis of the potential of zero charge of the metal surface and the adsorption ability of both Cl- and SO42- on the metal surface. (C) 2003 Published by Elsevier B.V.