This work presents a correlation of the oxidative behavior of formic acid to various Pt deposits on Au and bulk Pt in the absence and presence of Bi, studied by electrochemical means and X-ray photoelectron spectroscopy (XPS). Deposition of Pt on Au was performed using irreversible adsorption of Pt precursor ions, followed by electrochemical reduction, and a strategy of multiple Pt deposition was employed to control the deposited amount of Pt. Modification of the surfaces of Pt deposits and bulk Pt with Bi was carried out using irreversible adsorption of Bi. The amount of deposited Pt on Au, measured using coulometry and XPS, implied that most of Pt deposits are islands consisting of Pt monolayers partially covered with the second Pt layer. On such Pt deposits on Au, formic acid oxidation was enhanced by suppression of dehydration path and promotion of dehydrogenation path. In the presence of Bi on Pt deposits, dehydrogenation path was boosted further to increase the electrocatalytic activity by a factor of 4 below 0.4 V. On the other hand adsorbed Bi of coverage 0.25 on bulk Pt completely inhibited dehydration path and enormously sped up dehydrogenation path to be best among the scrutinized Pt surfaces. The enhancement factor of Bi-modified bulk Pt was 3 times compared to Bi-modified Pt deposits. The roles of Bi on Pt were discussed in terms of ensemble effect to suppress dehydration path and electronic effect to promote dehydrogenation path. In addition, electronic effect was found strongly to depend on the Pt thickness underneath adsorbed Bi. (C) 2016 Elsevier Ltd. All rights reserved.