Unlike energy efficiency and selectivity challenges, the kinetic effects of impure or intentionally mixed CO2feeds on the catalytic reactivity of the direct electrochemical CO2reduction reaction (CO2RR) have been poorly studied. Given that industrial CO2feeds are often contaminated with CO, a closer investigation of the CO2RR under CO2/CO co-feed conditions is warranted. Here, we report mechanistic insights into the CO2RR reactivity of CO2/CO co-feeds on Cu-based nanocatalysts. Kinetic isotope-labelling experiments—performed in an operando differential electrochemical mass spectrometry capillary flow cell with millisecond time resolution—showed an unexpected enhanced production of C2H4, with a yield increase of almost 50%, from a cross-coupled 12CO2–13CO reactive pathway. The results suggest the absence of site competition between CO2and CO molecules on the reactive surface at the reactant-specific sites. The practical significance of sustained local interfacial CO partial pressures under CO2depletion is demonstrated by metallic/non-metallic Cu/Ni–N-doped carbon tandem catalysts. Our findings show the mechanistic origin of improved C2product formation under co-feeding, but also highlight technological opportunities of impure CO2/CO process feeds for H2O/CO2co-electrolysers.