The CO electro-oxidation reaction has been studied on Rh[n(111) x (111)]-type electrodes in 0.5 M H2SO4 using chronoamperometry. The transients recorded on Rh(I 1 1), Rh(5 5 4), Rh(5 5 3) and R-h(3 3 1) are characterized by a current plateau, visible directly after charging of the double layer, followed by a main oxidation feature. which consists of two peaks, a pre-peak and a main peak. The current density in the plateau region is nearly constant over time and. thus, is of (quasi) zeroth order in CO coverage. A plot of log(j(plateau)) vs. E-final gives a linear relationship with a slope of ca. 45 mV dec(-1), suggesting a second electron transfer as the rate determining step. Analogously to platinum, the current density plateau was ascribed to a Langmuir-Hinshelwood type reaction between COads and OHads with no effective freeing of sites for OH adsorption due to relaxation of the CO adlayer. The presence of two peaks, rather than one, in the main oxidation feature can be explained by assuming a low surface mobility of CO and high oxidizability of rhodium surfaces. Indeed, dual step chronoamperometry shows that the mobility of CO on rhodium surfaces in aqueous media is very low. Since rhodium surfaces are known to oxidize readily, the pre-peak and main peak can be ascribed to CO reacting with OH, which adsorbs fast at the steps and more slowly at terrace sites. Since the geometry of the steps is nearly the same on each surface, the pre-peak appears structure insensitive. while the main peak shifts considerably with the step density. Introducing randomly distributed crystalline defects by cycling the electrodes repeatedly up to the surface oxidation region prior to each potential step experiment, results in a negative shift of the main peak, while the position of the pre-peak remains fixed. From the data presented, we conclude that the reaction nucleates at the steps and grows in the direction of the terraces. (C) 2004 Elsevier B.V. All rights reserved.