Using Pt(111) surfaces covered by ruthenium by a spontaneous deposition method we conducted voltammetric oxidation of chemisorbed CO in clean sulfuric acid electrolyte at 50 mV s(-1) and under variable sweep rate conditions. The deposition yields Pt(111) surfaces decorated by two-dimensional nanosized ruthenium islands previously reported. At 50 mV s(-1) the CO stripping produces two well-resolved current peaks, at 0.55 and 0.67 V versus RHE. The first peak originates from CO chemisorbed on (and strictly around) ruthenium islands deposited on Pt(111), while another comes from CO chemisorbed on 'pure' Pt(111) phases of the Pt(111)/Ru electrode, away from the Ru islands. Using double-potential step chronoamperometry we investigated the CO oxidation on these two surface phases, and found that the low potential oxidation of chemisorbed CO occurred via the Langmuir-Hinshelwood mechanism. This shows that diffusion of CO on ruthenium to the Ru edge is not needed for a complete stripping of CO from such islands, i.e. the oxidation process is activated uniformly across an island. In contrast, CO chemisorbed on Pt sites not occupied by Ru is oxidized at the Pt(111)/Ru edge, and there may be a surface diffusion contribution to a complete removal of CO from the surface. However, a part of the CO adlayer oxidation in the 0.67 V peak area occurs with no participation of surface diffusion, according to a pseudo-first order surface reaction kinetics, and the reaction may involve a 'reactant pair' (Pt-O-H...O=C=Pt) breakdown on the electrode surface.