Oxide formation reaction current density on Pt (1 1 1) was estimated by time-differentiating oxide growth curves during a potential hold as a function of the potential and the coverage with the oxide below 0.35 ML (monolayer). The Tafel slope was 18-24 mV decade(-1) in the condition where the Tafel plot is on a straight line. The reaction mechanism accounting for this small slope was proposed, in which the oxide formation process is assumed as a combination of a fast electron transfer step and a following slow step. (C) 2012 Elsevier Ltd. All rights reserved.