Scanning tunneling microscopy (STM) was used to examine spontaneously deposited ruthenium adlayers on the well-defined Pt(111) electrode. Stable and STM discernible structures were obtained after a brief cyclic voltammetric treatment of the ruthenium deposit. As demonstrated previously, the electrochemically stabilized Pt/Ru electrodes are active catalysts in methanol electrooxidation. The STM data indicate that the deposit is arranged in nanometer size islands of which the detailed structural characterization is presented. Maximum ruthenium coverage is no higher than 20%, which confirms our previous results obtained by the use of Auger electron spectroscopy. While most of the islands are monatomic, a fraction of the islands, approximately 10% of the total ruthenium coverage, displays a second monolayer deposit over the first monolayer. This is a new discovery showing an unexpected tendency of the spontaneously deposited ruthenium at such a low coverage to nucleate in a bilayer configuration. The hydrogen underpotential deposition process does not affect the spatial distribution of the islands, but ruthenium is reductively removed from the surface under hydrogen evolution conditions.