The electrodeposition of Hg on polycrystalline smooth and columnar structured Pt (cs-Pt) substrates was comparatively studied in acid solution at 298 K applying potentiodynamic techniques. The topography of both types of Pt electrodes was characterized by scanning tunneling microscopy. The use of cs-Pt substrates provides information about surface processes which are not easily detected on conventional smooth Pt. At moderate and high potential scan rates Hg electrodeposition under mass transport control on cs-Pt yields a non-equilibrium complex deposit consisting of 2d and 3d Hg domains, and Pt-Hg alloy coexisting with bare Pt domains at intercolumnar voids. The dynamics of these domains approaching equilibrium can be followed by conventional voltammetry. The kinetics of Hg penetration into bulk Pt results from a Hg atom shift to a position underlying the first layer of Pt atoms followed by a diffusion-controlled penetration into bulk Pt. The Hg atom diffusion coefficient into bulk Pt results in D≅2×10⁻¹⁷ cm² s⁻¹Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas