The variation in CO adsorption structures during the preoxidation of CO on Os-modified Pt(111) (Pt(111)/Os) was investigated using cyclic voltammetry and electrochemical scanning tunneling microscopy. The spontaneous deposition of Os on Pt(111) resulted in randomly scattered islands with a coverage range of 0.13-0.54. During preoxidation on Pt(111)/Os, a phase transition from (2 x 2)-alpha to (root 19 x root 19) via the transient structures of (2 x 2)-beta and (1 x 1) took place as on unmodified Pt(111). As the amount of Os increased, however, the transient structures of (2 x 2)-beta and (1 x 1) appeared at lower potentials with higher populations. When the population of the transient structures was greater than 50%, an oxidative CO stripping process took place to the structure of (root 19 x 9), completing the preoxidation. These observations strongly support the idea that the presence of Os increases the mobility of adsorbed CO by electronic modification of the Pt(111) surface (electronic effect). In addition, the results obtained with Pt(111)/Os were compared with those of Pt(111)/Ru.