A closer look at the surface speciation of these two HCOO -selective bimetallic systems (Cu25In75 and Cu40Sn60) reveals the fully metallic nature of all elements at the potential with the best HCOO -production (-0.9 V). Despite their similar speciation and CO2ER performance at -0.9 V, they displayed a completely different CO2ER behavior at lower potentials (see Figure and), with Cu25In75 yielding mainly CO at low potentials (<-0.7 V) while Cu40Sn60 produces mostly H2. These differences in the major products at low potentials may originate from the dif ferences in their surf ace speciation (Figure and Figure ). Where the f ully metallic surface of Cu25In75 seems to have a high tendency f or CO production with little H2 f ormation, partially oxidized Sn surface of Cu40Sn60 f avored H2 production over CO at low potentials. Scheme 1&2 summarize the surface-active species of CO-and HCOO-selective bimetallic catalysts, as evaluated from quasi in-situ XPS, and depict our theory of how they are related to the observed selectivity switch from CO f or Cu-rich to HCOO-f or Cu-poor bimetallic catalysts. The enhanced CO-production at Cu-rich system can be attributed to the localized partial negative charge on Cu sites due to the charge transfer f rom Sn/In atoms to Cu. This charge transfer from Sn/In into Cu led to partial positive charge on Sn/In atoms, indicated by the oxidized Sn and In species, and hence stabilizing the *COOH intermediate (CO-pathway). Within the resolution of our XPS analysis we do not observe clear evidence of charge transfer, but rather both metals are f ully reduced on the surf ace. Thus, Cu-poor catalysts (HCOO-selective) is thought to enhance the HCOO-pathway by inhibiting the H-adsorption and stabilizing the *OCHO intermediate (HCOO-pathway), see in-situ SERS below.