Underpotential deposition (upd) of zinc ions is observed on Pt(1 1 1) in alkaline solution as a sharp cyclic voltammogram (CV) peak. in contrast re, upd on polycrystalline Pt, for which several broad upd peaks are observed. The CV characteristics on Pt(1 1 1) were studied from changes in the sweep rates. zincate ion concentrations ZnO22-, and pH. The Zn upd formation/desorption mechanism is analyzed and discussed in terms of a nucleation-growth mechanism. Changes in the upd peak potential with changes in the concentration of ZnO22- and OH were 60 mV/log[ZnO22-] and - 180 mV/pH. The former value gives an apparent electron transfer number n(a) = 1. The OH-adsorption/desorption peak on Pt(1 1 1) shifted with - 60 mV/pH and CV peak width 90 mV, independent of sweep rate. These results show that the reaction OH- = OHads + e(-) takes place according to the Langmuir adsorption mechanism. The different pH dependences for Zn upd and OH- adsorption/desorption processes suggest that the electrode reaction in alkaline solution. ZnO22- + 2H(2)O +2e(-) = Zn + 4OH(-). is involved in upd on Pt(1 1 1). The value n(a) = 1 for the ZnO22- upd process can be discussed in terms of two-electron transfer. such that Zn2+ is reduced to a Zn-0 state at Pt(1 1 1) with a true electron transfer number of n = 2, and the induced OH-adsorption takes place as OHads onto upd Zn at Pt(1 1 1) by an oxidative process. The overall upd process can be expressed as ZnO22- + 2H(2)OP + e(Pt(1 1 1)) reversible arrow OH\Zn\Pt + 3OH(-) which is in good accord with the observed pH dependence of - 180 mV/pH. Consequently. the apparens electron transfer number n(a) becomes 1. From the charge densities of the upd laver and the OH layer, the coverage ratio of OHads/Zn-upd is equal to 0.59/0.45 with respect to surface Pt atoms.