The electroreduction of hydrogen on Pt(III) in acidic media is the primary fuel cell reaction. The explicit mechanism for this process involves a first order phase transition, which is due to the rotation of the water dipole as the electric field is changed. For positive electrodes the water forms linear 'polywater' chains that run between the adsorbed bisulfate ions. This phase is seen by STM experiments. When the electrode turns negative, then a honeycomb structure is formed by the reaction H(3)O(+) + H(2)O = H(5)O(2)(+) The structure of this intermediate compound is studied using high accuracy generalized gradient PW91 calculations for a 5 layer Pt slab