The electroformation of adsorbed species on Pt and Rh from CO2 dissolved in aqueous 0.5 M H2SO4 and the influence of adsorbed species on the hydrogen evolution reaction (HER) were examined for smooth and columnar structured Pt and smooth and rhodized Rh electrodes at 25 degrees C. Different electrosorbed species resulting from the adsorption of CO2 on Rh were found. The electro-oxidation of these adsorbates on Rh takes place over a potential range larger than that for adsorbates on Pt, overlapping the potential range of electroformation of the O-containing layer. A part of the reduced CO2 adsorbates on Rh behaves as CO-like adsorbates. The presence of adsorbed species from CO2 influences differently the stationary HER current potential curves for both types of Pt and Rh electrodes. In the range of low current density (cd), the Tafel slope for Pt is - 0.030 V decade(-1). Conversely, the presence of a chemisorbed species on Rh changes the Tafel slope for the HER in the range of low cd from -0.030 V decade(-1) for CO2-free solutions to -0.120 V decade(-1) for CO2-saturated solution. These changes are explained by a modification in the rate-determining step of the HER produced by chemisorbed species.