Abstract The coadsorption of hydrogen and anions on smooth polycrystalline platinum has been studied by the light electroreflection and reflection phase electromodulation methods. The plasma electroreflection spectrum has been calculated. It has been shown that in HCl and HBr solutions in a wide potential range, the change in the optical properties of the interface follows the plasma mechanism. The dependences on the potential of the differential true residual charge on the Cl and Br adatoms has been determined and the simultaneous existence of halide species with different residual charges on the surface has been proved. Quantitative data on the adsorption of hydrogen and the anions ClO − 4, SO 2− 4, Cl − and Br − on smooth platinum have been obtained and compared with the data of the electrochemical and radiotracer methods.