Rhodium adlayers on Pt(1 1 1) substrates have been prepared by electrodeposition from dilute Rh3+ acidic solutions. Resulting deposition rates are lower than 0.03 ML min(-1). Pseudomorphic growth of the first monolayer has been confirmed by scanning tunneling microscopy (STM) as well as the formation of small compact islands in the submonolayer range. Carbon monoxide oxidation and nitrous oxide reduction have been studied on Rh/Pt(1 1 1) electrodes. The oxidation of carbon monoxide is catalyzed by the presence of very low coverages of rhodium as demonstrated by the negative shift of the CO oxidation profile. Results are compatible with a bifunctional mechanism for catalysis including CO diffusion in the Pt domains toward the edges of the islands (splitting of the voltammetric oxidation profile). The reduction of nitrous oxide occurs at different potential and with different rates on Pt domains, at the center of the Rh islands and at their edges, being the latter sites especially active. In any case, the adsorptive and catalytic activity of the adlayers differ from those of the bulk Pt(1 1 1) and Rh(1 1 1) electrodes. The existence of strain in the film together with a diminution in the coordination number for adatoms at the edges of the islands are considered to be at the origin of the observed behavior. (C) 2003 Elsevier Ltd. All rights reserved.