Though Lewis acids (LAs) have been shown to have profound effects on carbon dioxide (CO2) reduction catalysis, the underlying cause of the improved reactivity remains unclear. Herein, we report a well-defined molecular system for probing the role of LA additives in the reduction of CO, to carbon monoxide (CO) and water. Mo(0) CO, complex (2) forms adducts with a series of LAs, demonstrating CO2 activation that correlates linearly with the strength of the LA. Protons induce C-O cleavage of these LA adducts, in contrast to the C-O, displacement primarily observed in the absence of LA. CO2 cleavage shows dependence on both bond activation and the residence time of the bound small molecule, demonstrating the influence of both kinetic and thermodynamic factors on promoting productive CO2 reduction chemistry.