Presented is a voltammetric study of co-adsorption of irreversibly adsorbing Sb and Te on Pt(111). When a layer of Sb and Te was formed via simultaneous adsorption, the reduction peak of Te was observed at 0.30 V in the initial cathodic scan. In contrast, sequential adsorption of Sb followed by Te adsorption led to a Te reduction peak at 0.50 V in the initial scan. As the voltammetric scan was continued, in addition, the voltammogram of the simultaneously co-adsorbed layer changed, while that of the sequentially co-adsorbed layer did not. These observations are discussed in terms of formation of a homogeneously mixed layer and a layer consisting of heterogeneously separated domains of Sb and Te. Also, the difference in the adsorption strength of Sb and Te was observed.