The influences of the Cu adatoms and Cu+ ions on the initial stages of copper electrodeposition on polycrystalline Pt in acidified copper sulphate solution have been studied by cyclic voltammetry, potentiostatic current transients, rotating ring-disk electrode (rrde), and in situ scanning tunnelling microscopy (STM). It has been established that during the long-time polarization of the platinum electrode, at potentials close to the copper equilibrium potential, the concentration of Cu+ ions in the vicinity of the electrode increases and a Cu microdeposit if formed. The dissolution of the microdeposit occurs very slowly and takes several minutes of potential cycling in the range of 0.27-1.25 V. While the microdeposit is present on the platinum, the electrode surface is more active with respect to upd and bulk copper deposition. The combination of electrochemical and in situ STM results indicates that formation of surface alloy occurs during electrodeposition of copper on polycrystalline platinum.