Schemes 1 and 2 summarize our observations of the surf ace-active species of CO-and HCOO --selective bimetallic catalysts (respectively), as evaluated from quasi in-situ XPS, and hypothesized general mechanisms of how the speciation relates to the observed selectivity switch f rom 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 from Sn/In atoms to Cu. This charge transfer from Sn/In into Cu led to partial positive charge on Sn atoms, indicated by the observing oxidized Sn and In species, and hence stabilizing the *COOH intermediate (CO-pathway). 17, 24