We propose a new mechanism for the electroreduction of hydrogen in acidic media. The specific model and theoretical calculations are designed for the Pt(111) surface, because this is the only face that exhibits a clear first order phase transition. The basic mechanism however, applies to the other faces of Pt as well. The principal ingredient of our theory is the assumption that the water molecules in the inner Helmholtz layer are strongly oriented by the field. This means that hydrogen bonds have to be bent considerably. A correct water model has to be tetrahedral and flexible. Then, as the polarity of the electrode is changed: For positive electrodes most of water dipoles point down, and a zig-zag chain of hydrogen bonded water is formed which is responsible for the root3- x root7 (or perhaps root3- x 5/2) structure of the sulfate observed by STM. For negative polarization most of the water dipoles point up. The (bi)sulfate, and the water form a periodic structure by incorporating 1/3 of hydronium, which then is adsorbed with a hydrogen down in the hollow sites of the Pt(l 11) electrode. This compound is neutral and forms a two dimensional honeycomb arrangement of water with bisulfate ions. When the electrode becomes more negative, then the bisulfate is desorbed, but the honeycomb structure remains because of the reaction (H5O2+), + 6e(-) - 3H(2) + (H3O2-)(3) (1) This reaction is studied by a high accuracy generalized gradient PW91 calculation for a five layer Pt slab. The HER or intermediate reaction compound (H3O2-)(3) is stable on the Pt(111) face. The model is consistent with all known experiments: it reproduces well the experimental voltammogram and the recent radiotracer measurements of bisulfate adsorption. Furthermore, it gives a robust explanation of the 2/3 hydrogen yield for this surface. (C) 2002 Elsevier Science B.V. All rights reserved.