Effects of ethoxylated alpha-naphtholsulfonic acid (ENSA) on the initial stages of tin plating have been studied on iron electrodes in an acidic stannous sulfate solution containing phenolsulfonic acid as a supporting electrolyte using potentiodynamic polarization, electrochemical quartz crystal microbalance (EQCM), scanning probe microscopy (SPM), and electrochemical impedance spectroscopy techniques. The smallest exchange current density and a larger transfer coefficient are observed at a typical ENSA concentration used in industrial plating baths, i.e., 0.013 M. The SPM imaging and EQCM measurements show that ENSA molecules form a compact structure by interacting with neighboring molecules at the iron surface, which controls the mass transport for Sn(II) reduction. The EQCM studies indicate that the ENSA molecules remain stably adsorbed on the electrode surface at considerably high overpotentials. The ENSA molecules present in both the tin layers and the solution are found to slow the hydrogen evolution reaction at, as well as the corrosion process of, the tin-plated electrode, acting as an anticorrosion agent in commercial tin plating baths. (C) 2004 The Electrochemical Society.