The development of earth-abundant catalysts for selective electrochemical CO2 conversion is a central challenge. CuSn bimetallic catalysts can yield selective CO2 reduction toward either CO or formate. This study presents oxide-derived CuSn catalysts tunable for either product and seeks to under-stand the synergetic effects between Cu and Sn causing these selectivity trends. The materials undergo significant transformations under CO2 reduc-tion conditions, and their dynamic bulk and surface structures are revealed by correlating observations from multiple methods—X-ray absorption spectros-copy for in situ study, and quasi in situ X-ray photoelectron spectroscopy for surface sensitivity. For both types of catalysts, Cu transforms to metallic Cu0under reaction conditions. However, the Sn speciation and content differ significantly between the catalyst types: the CO-selective catalysts exhibit a surface Sn content of 13 at. % predominantly present as oxidized Sn, while the formate-selective catalysts display an Sn content of ≈70 at. % consisting of both metallic Sn0 and Sn oxide species. Density functional theory simula-tions suggest that Snδ+ sites weaken CO adsorption, thereby enhancing CO selectivity, while Sn0 sites hinder H adsorption and promote formate produc-tion. This study reveals the complex dependence of catalyst structure, compo-sition, and speciation with electrochemical bias in bimetallic Cu catalysts.