The corrosion inhibition of iron in 0.5 M H2SO4 Solutions by alkyl quaternary ammonium halides (AQAH) inhibitors has been studied by potentiodynamic polarization curves and electrochemical impedance spectroscopy (EIS) measurements. The correlation between inhibition efficiency and molecular structure of the AQAH compounds is investigated. The results show that besides the concentration, the structure of alkyl groups and the type of halide ions of these AQAH inhibitors greatly influence the inhibition efficiency. Data obtained from EIS measurements are analyzed to model the corrosion inhibition process through appropriate equivalent circuit models. (c) 2005 Elsevier B.V. All rights reserved.