Dissociative adsorption of formic acid on adatom modified Pt(100) electrodes was carried out as a way of studying inhibition of the poison formation reaction on these surfaces. The electrodes were prepared using irreversible adsorption of bismuth and antimony. The dissociative adsorption method employed throughout this work leads to an improvement in surface control with respect to that previously used. Results show that bismuth and antimony adsorbed on Pt(100) electrode inhibit poison formation through a third body mechanism. Poison species (CO(ads)) can be isolated and characterized in the high coverage range as a consequence of the dissociative adsorption technique employed.