Electrochemical AFM is a powerful tool for the real-space characterization of catalysts under realistic electrochemical CO reduction (CO RR) conditions. The evolution of structural features ranging from the micrometer to the atomic scale could be resolved during CO RR. Using Cu(100) as model surface, distinct nanoscale surface morphologies and their potential-dependent transformations from granular to smoothly curved mound-pit surfaces or structures with rectangular terraces are revealed during CO RR in 0.1 m KHCO . The density of undercoordinated copper sites during CO RR is shown to increase with decreasing potential. In situ atomic-scale imaging reveals specific adsorption occurring at distinct cathodic potentials impacting the observed catalyst structure. These results show the complex interrelation of the morphology, structure, defect density, applied potential, and electrolyte in copper CO RR catalysts.