It is crucial to understand the electrochemical HCIO4/Pt(111) interface in the field of electrochemistry, which is unfortunately still disputed. To reveal the fundamental properties of this interface, we have studied co-adsorption of possible species at the HCIO4/Pt(111) interface using density functional theory (DFT). Our calculations demonstrate that HClO4* is most likely to dissociate into CIO4* on Pt(111) surface, leading to an adlayer of ClO4* co-adsorbed with H3O* and H2O* molecules. Interestingly, these co-adsorbed H3O* molecules exhibit the nature of hydronium ions with highly localized positive charge states. On the other hand, the stability of this ClO4* co-adsorbed adlayer is almost equal to that of a H2O* + H3O* bilayer. This competition would prohibit the formation of periodic CIO4* + H2O* + H3O* adlayer on Pt(111), yielding a weak adsorption of CIO4* in electrochemical condition as the experiments found. In addition, the electrostatic potential along the direction that is perpendicular to the substrate exhibits clearly distinguishable peaks for the different interfacial structures. These results provide deep insights into this fundamental electrochemical interface and offer promising clues for future experimental studies. (C) 2016 Elsevier B.V. All rights reserved.