The deposition of As on a Pt(l 11) electrode has been studied by cyclic voltammetry and scanning tunneling microscopy (STM). The shape of the redox peaks of As is very much influenced by its surface coverage: At partial coverages, which can be achieved by spontaneous deposition at open circuit, a single sharp, reversible peak is observed at 0.57 V in addition to the hydrogen and sulfate adsorption peaks, which somewhat broadens when hydrogen adsorption is completely suppressed; further deposition of As leads to an additional pair of peaks at 0.38 or 0.45 V (cathodic or anodic sweep direction, respectively). Whereas a monolayer of As deposited at open circuit or above 0.45 V can be dissolved, again leaving behind a smooth Pt(111) surface, deposition below 0.38 V, and in particular at potentials where bulk As deposition occurs, leads to surface roughening. In cyclic voltammetry, the typical shape of hydrogen and sulfate adsorption peaks is not regained, and STM images reveal a rough surface after dissolution of As. We interpret this as being due to a surface alloying process which takes place when the As coverage exceeds a certain, critical value. Bulk As deposition leads to 3D deposits. Tip-induced nanostructuring of As at Pt(l 11) can be achieved at more positive potentials in an As3+-containing solution where no surface roughening takes place. The structures are formed due to local alloy formation when the tip is scanning at a very short distance above the Pt(111) electrode.