Irreversible adsorption of oxocarbons on bismuth and selenium modified Pt(lll) electrodes has been examined by cyclic voltammetry and in situ FT-IR spectroscopy. A simple third-body effect is observed in the adsorption of those oxocarbons giving rise to only one adsorbed residue : squaric acid which is dissociatively adsorbed to CO adsorbates and tetrahydroxy-p-benzoquinone which is directly adsorbed on these surfaces. On the other hand, croconic and rhodizonic acids give rise to more than one adsorbed residue (CO adspecies and another adsorbate with more than one carbon atom) and thus, particular effects arising from their stability can be identified, besides the third-body effects appearing at high adatom coverages. Then, bismuth ad-atoms promote the surface cleavage of croconic acid adsorbates to CO adspecies and selenium ad-atoms hinder the adsorption of some reaction intermediate involved in the conversion of this residue to CO adspecies. FT-IR measurements reinforce these voltammetric results. In the ca se of rhodizonic acids, whose adsorption intermediates a re CO and tetrahydroxy-p-benzoquinone, both ad-atoms promote the formation of CO adspecies, being this effect emphasized more on Pt(lll)-Se electrodes.