In summary, the complementary information gained f rom the various spectroscopic and microscopic techniques in ex-, quasi in-situ or operando approaches allows us to fully examine the origin of the selectivity switch for the bimetallic Cu-M (M=In or Sn) catalysts f rom CO to HCOO -upon increasing In or Sn contents. The inf ormation gathered f rom these techniques enabled us to understand the significant differences in the surf ace speciation and composition of between Cu-In and Cu-Sn systems, despite both systems showing similar CO2ER perf ormance. Our f indings from in-situ SERS highlight the importance of the developed local alkalinity in the f ormation of oxidized surface Cu species, where a mixture of Cu hydroxide and basic Cu carbonate (metastable malachite like materials) was observed.