The adsorbate structures on electrode surfaces in an electrochemical environment are controlled by thermodynamic parameters such as temperature, concentration, pH and electrode potential. Knowledge of these structures is important as specifically-adsorbed ions on an electrode impact catalytic reactions that take place at the electrode-electrolyte interface. From a theoretical point of view, the equilibrium structures of adsorbates can be conveniently estimated using the concept of the computational hydrogen electrode. Here we extend this concept to determine equilibrium co-adsorption structures of halides with hydrogen on Pt(111) as a function of the corresponding electrochemical potentials. We find that hydrogen-halide co-adsorption is of a competitive character, which means that mainly dense-packed structures of either halides or hydrogens are stable on the surface, in good agreement with experiment.