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The presence of an external magnetic field is found to affect the competition between the H 2 O and CO 2 reduction reactions by increasing mass transport via the Lorentz force. Increasing the magnetic field strength at the electrode surface from 0 to 325 mT increases the selectivity of CO over H 2 by 3×, while an increase in current density from 0.5 to 5 mA/cm 2 increases the selectivity of CO production by 5×. Cyclic voltammetry and finite-element simulations reveal that the origin of the enhanced CO selectivity is attributable to a magnetic field lowering the electrode-electrolyte interfacial pH. A drop in interfacial pH enables increased production of CO from CO 2 reduction due to a decrease in the activity of H 2 O reduction and increase in CO 2 solubility near the electrode surface. The insight provided in this study offers new opportunities to control reaction selectivity in electrocatalysis with magnetic field vectors.