Electrochemical adsorption of tin on Pt(111) was studied by cyclic voltammetry and scanning tunneling microscopy. At high coverages, Sn adlayers show only one broad oxidation peak at 0.8 V. With decreasing Sn coverage, a specific redox peak pair appears at around 0.61 V and achieves a maximum value of 76 μC cm⁻². Its decrease upon further dissolution of Sn is connected to the appearance of hydrogen and sulfate adsorption. The coexistance of the ‘spike’ for sulfate adsorption with the Sn redox peaks at low coverages indicates the segregation of Sn into islands. Tin islands could only be observed by STM after coadsorption of CO or Cu-UPD. Besides decoration of steps, tin islands were randomly dispersed on the terraces of the Pt(111) electrode surface. After oxidation of CO the Sn domains are no longer visible at potentials in the hydrogen region. The reason is the high mobility of Sn on the Pt(111) surface.