The irreversible adsorption of selenium on Pt(111) electrodes allows Pt(111) modified electrodes with a selenium coverage up to 0.5 to be prepared and characterized voltammetrically. Such modified electrodes have different electrocatalytic properties for formic acid oxidation than the unmodified Pt(111) electrode. Two different aspects of the formic acid oxidation reaction were studied : (i) the catalytic enhancement of formic acid oxidation with selenium coverage and the influence of the surface structure of the selenium adlayer and (ii) the inhibition of the poison formation reaction from formic acid by the presence of adsorbed selenium on the electrode surface. An increase in the catalytic activity of the modified electrode was found at selenium coverages greater than 0.20, maximum catalytic activity being achieved at a selenium coverage of 0.28. With selenium coverages higher than 0.33, the catalytic activity of the electrode diminishes. In the poison formation experiments, the poison formation reaction was inhibited by the selenium adatoms by a third-body mechanism. The complete inhibition of the poison formation reaction occurred at a selenium coverage of 0.28, the coverage at which the maximum catalytic activity was found. A relationship between the structure of the selenium adlayer and the catalytic enhancement is proposed.