We report here on an electrochemical study of carbon dioxide electroreduction (CO 2 ER) employing electrochemical impedance spectroscopy (EIS) and a rotating ring-disk electrode (RRDE) on different gold based catalysts, namely bulk polycrystalline gold, gold nanoparticles (Au NPs) and Au@Ag core–shell nanoparticles. RRDE measurements allowed the identification of the reduction potential of CO 2 to CO and the characterisation of NPs based on their selectivity for CO 2 ER with respect to hydrogen evolution. Gold–silver combined catalysts were found to be more selective than Au NPs although at higher overpotential. One gold–silver catalyst exhibits a constant selectivity over a wide potential range. For the first time, EIS data showed the existence of two charge transfers during the reduction of CO 2 . These are attributed to two surface confined reactions that involve an adsorbed intermediate which is correlated with a proposed mechanism. The potential range of these reactions is specific for each catalyst with gold–silver catalysts exhibiting a slower electron transfer than Au NPs, Au and Ag polycrystalline electrodes.