The initial stages of Ni deposition on {111} textured Au electrodes in dimethyl sulfoxide (DMSO) solutions were studied by a combination of electrochemical measurements and ex situ X-ray photoelectron spectroscopy analysis. Ni deposition occurs in two steps: underpotential deposition (UPD) of ca. one monolayer of Ni at potentials positive of the Nernst potential, followed by bulk Ni deposition. The mechanism of deposition of the Ni UPD layer differs from the usual UPD in that it is accompanied by a chemical reaction of Ni adatoms with DMSO and formation of elemental sulfur on the electrode. The sulfur adlayer remains on the electrode surface after dissolution of the Ni overlayer and facilitates the kinetics of subsequent Ni deposition. Anodic dissolution of the Ni UPD layer after multiple depositions proceeds in two steps, suggesting the formation of a Ni/Au surface alloy. (c) 2005 The Electrochemical Society.