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We report herein the in situ electrochemical scanning tunneling microscopy (ECSTM) study on the synergistic effect of Mg 2+ in CO 2 reduction reaction (CO 2 RR) catalyzed by cobalt phthalocyanine (CoPc). ECSTM measurement molecularly resolves the self-assembled CoPc monolayer on the Au(111) substrate. In the CO 2 environment, high-contrast species are observed in the adlayer and assigned to the CO 2 adsorption on CoPc. Furthermore, the contrast of the CO 2 -bound complex is higher in Mg 2+ -containing electrolytes than in Mg 2+ -free electrolytes, indicating the formation of the CoPc-CO 2 -Mg 2+ complex. The surface coverage of adsorbed CO 2 is positively correlated with the Mg 2+ concentration as the additive in electrolytes up to a plateau of 30.8 ± 2.7% when c(Mg 2+ ) > 30 mM. The potential step experiment indicates the higher CO 2 adsorption dynamics in Mg 2+ -containing electrolytes than without Mg 2+ . The rate constants of CO 2 adsorption and dissociation in different electrolytes are extracted from the data fitting of statistical results from in situ ECSTM experiments.