Understanding the changes that a catalyst may experience on its surface during a reaction is crucial in order to establish structure/composition-reactivity correlations. Here, we report on bimetallic size-selected Cu_100-xCo_x 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*.