The structure and orientation of water molecules on Au(1 1 1-20nm) film electrodes in contact with aqueous sulphuric acid solution was studied by surface enhanced infrared reflection-absorption spectroscopy (SEIRAS) employing an attenuated total reflection (ATR) configuration (ATR-SEIRAS) with a vertical spectroelectrochemical cell. The spectrum of interfacial water is strongly dependent on electrode potential, ionic strength and pH. Coadsorption of hydronium ions with weakly hydrogen-bonded water molecules was found at E < E-pzc. At E > E-pzc strong hydrogen-bonding among water molecules and to coadsorbed sulphate species exists. Based on pH-dependent electrochemical, spectroscopic and in situ STM investigations, a new model is suggested to represent the ordered (root3 x root7) adlayer at maximum sulphate coverage theta = 0.2: a Zundel or hydrated hydronium ion H5O2+, in which one proton is shared between two water molecules, bridges adjacent sulphate species via hydrogen-bonds along the main diagonal of the (root3 x root7) unit cell. This alternating arrangement gives rise to a long-range ordered, 2D network of sulphate and water species interconnected by hydrogen-bonds, and capable to form hydrogen-bonds with second-layer water species. The suggested model is consistent with all experimental observations as well as predictions from quantum-chemical and molecular dynamic simulations. (C) 2003 Elsevier Ltd. All rights reserved.