The surf ace composition and speciation of Cu-based bimetallic catalysts (Cu-Sn and Cu-In) have substantial ef f ects on their performance (selectivity) for CO2ER, as discussed above. However, most of the previous studies build their mechanistic hypotheses based on structural inf ormation gathered f rom ex-situ measurements conducted either before or af ter CO2ER testing. Since it is well known that catalyst materials can significantly transf orm under CO2ER measurement conditions, basing DFT calculations on such structural information as a main way to explain the observed selectivity trends of these bimetallic systems can be challenging and potentially misleading, as the models likely do not represent the precise active-surface species. Accordingly, indepth exploration of the surface chemical environment, speciation (oxidation states), structure and composition of those bimetallic systems using combination of appropriate and complementary surface-sensitive techniques via operando/in-situ configurations is required to fully expose the complex link between their composition, intermediate adsorption, and selectivity. As mentioned above, limited number of studies have reported the high selectivity of Cu-In catalysts into either CO or HCOO -, but none of them, to the best of our knowledge, provide detailed insight into the active surface speciation. Furthermore, while one may presume that Cu-In and Cu-Sn systems follow the same mechanisms for selectivity tuning based on the similar composition-activity trends, to date no study has systematically compared the two to address this possibility . Thus, there is a need for detailed and systematic investigations of the surface properties of such bimetallic catalysts to define the precise surfaceactive species, which govern their selectivity shift from CO to HCOO -. This work is mainly dedicated to provide a thorough structure-composition-activity comparison between Cu-In and Cu-Sn bimetallic electrocatalysts using various surface and bulk sensitive X-ray spectroscopic techniques, aimed at providing insight into the open questions regarding the similarity and differences between those two systems. In this regard, multiple complementary techniques including quasi in-situ X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS) combined with in-situ surface enhanced Raman spectroscopy (SERS) were used to probe the chemical environment and surf ace changes under CO2ER electrolysis. A simple one-pot electrochemical synthesis strategy was used to prepare bimetallic Cu-In and Cu-Sn with dendrite-like structures across a range of controlled In and Sn contents to tune their selectivity towards either CO or HCOO-. In-situ SERS was used to probe the catalyst/electrolyte interface changes (e.g., local pH changes, surface oxide species) and key intermediates of CO2 reduction for a better understanding of the origin of the pref erence shift f rom C-bound intermediate to O-bound intermediate with increase of Sn/In surface contents. This comparative study highlights key differences in the surface composition and speciation (surfaceactive species) between Cu-In and Cu-Sn bimetallic systems under CO2 electroreduction conditions, despite both systems showing similar CO2ER catalytic activity behaviors. It also introduces new insights into the debated role of the persistence metal surface oxide species and their dynamic transformations during CO2ER.