The oxidation of formic acid at Pt electrodes in the presence of underpotentially deposited (UPD) Pb has been studied using an electrochemical quartz crystal microbalance (EQCM). Although the current associated with the UPD process is largely obscured by current from the oxidation of formic acid, the mass response is dominated by the changes in UPD coverage. Thus examination of mass responses accompanying cyclic voltammetric and constant-potential experiments reveals both variations in UPD coverage and the manner in which the underpotential deposits are affected by adsorbates derived from formic acid. At low concentrations of formic acid there is some suppression of the underpotential deposit and data suggest that strongly adsorbing intermediates form most rapidly in the hydrogen adsorption region of potential. Mass responses also indicate slight increases in UPD coverage upon removal of strongly adsorbed species by oxidation. Oxidation of high concentrations (0.1 M) of formic acid induces a significant positive shift in the potential for removal of the UPD deposit on the positive scan, and on the subsequent negative scan the rapid reaction between the oxidized Pt surface and formic acid removes the oxide at a higher potential than normal and consequently allows the UPD process to begin at a more positive potential. Adsorption of Pb2+ at oxidized Pt surfaces is also inhibited by the presence of formic acid.