Understanding the changes that a catalyst may experience on its surface during a reaction is crucial in order to stablish structure/composition-reactivity correlations. Here, we report on bimetallic size-selected Cu100-xCox nanoparticle (NP) catalysts for CO2 electroreduction reaction (CO2RR) and we identify the optimum Cu/Co ratio and NP size leading to improved activity and selectivity. Operando X-ray absorption spectroscopy (XAS) and quasi in situ X-ray photoelectron spectroscopy (XPS) provided insight into the morphological, structural, and chemical transformations underwent by the CuCo NPs during CO2RR. We illustrate that the as-prepared state of the bimetallic NPs is drastically different from the structure and surface composition of the working catalyst. Under electrochemical conditions, a reduction of both initially oxidized metallic species was observed, accompanied by Cu surface segregation. Density functional theory (DFT) results from a Cu3X model were used to describe the surface segregation. In order to extract mechanistic understanding, the activity of the experimental Cu and CuCo NPs towards CO2RR was described via DFT in terms of the interaction of Cu facets under expansion and compression with key reaction intermediates, in particular CO* and COOH*.