A simple model for simulating the electrooxidation of HCOOH on single crystal surfaces of Pt catalyzed by adatoms is presented. The substrate is represented as a square or hexagonal array of lattice points, which may be occupied by adatoms or poisoning species. Analytic expressions for the oxidation current as a function of the adatom coverage are given. Third body effects as well as real improvement of the catalytic activity are analyzed. The model fits satisfactorily some experimental data for the electrooxidation of formic acid on Pt/adatoms systems.