The initial steps of the electrochemical reduction of CO2 at Pt electrodes were computationally investigated at the molecular level. Simulations were performed with density functional theory using the B3LYP functional and effective core potential basis sets. The surface was modeled through two clusters comprising 13 and 20 atoms. An implicit solvation model was used to describe solvation effects for two different solvents: water and acetonitrile. It was found that CO2 adsorption is highly favored on negatively charged clusters and takes place passing from a well-defined transition state. The computational evidence suggests that the electrodic CO2 adsorption reaction may be described as a concerted process in which an electron-transfer reaction takes place contextually to CO2 adsorption. Also, the present results suggest that the formation of the CO2 center dot- aqueous species is significantly unfavored from an energetic standpoint and that its main fate, if formed, would be most likely that of getting adsorbed again on the Pt surface. The calculation of the pK(a) of adsorbed CO2- showed that its protonation reaction is thermodynamically favored in most electrochemical conditions used for CO2 reduction. Also, it was found that the free-energy difference between adsorbed formate and adsorbed COOH favors the latter, suggesting that the interconversion kinetics of these two species at a Pt surface may play an important role in determining the system reactivity. A tentative global mechanism able to describe the CO2 reactivity on Pt surfaces is proposed.