Transients of open-circuit potential are measured for the interaction of HCOOH (in 0.5 M H2SO4) with oxygen preadsorbed on a Pt/Pt electrode. A monolayer of O-ads (ML O-ads) in formic acid solutions is removed much more slowly as compared with CO solutions of equal concentrations. Potentiodynamic pulses show that the reduction of ML O-ads proceeds most slowly in the regions of great O-ads coverages (theta(0) >= 0.8) for HCOOH solutions and at medium coverages in CO solutions. The difference in the behavior of HCOOH and CO is explained by the fact that chemisorption is dissociative for HCOOH and associative for CO. In the region of medium theta(0), E versus tau dependences for both compounds studied are well described by an equation derived for the mechanism of "conjugated reactions" under the assumption that adsorption of HCO-compounds on sites unoccupied by O-ads is the limiting stage. (C) 2005 Elsevier Ltd. All rights reserved.