The nickel underpotential deposition (upd) process at a Pt electrode was studied using the cyclic voltammetric technique. In strong acidic medium, Ni upd takes place in the hydrogen adsorption-desorption region. The maximum coverage was found to be approximate to 0.55 monolayer at 10(-3) M Ni2+ ions in phosphate buffer (pH approximate to 6.85). The estimation of the electrosorption valence gave a value of ca. 2 with a replacement of one adsorbed hydrogen atom by each deposited Ni atom. The Ni upd coverage was found to depend on the surface structure of the Pt electrode. The oxidation of HCOOH was inhibited by Ni upd and the inhibition effect became more pronounced as the pH increased.