The electrochemical reduction of CO2 has drawn a large amount of attention due to its potential to produce sustainable fuels and chemicals by using renewable energy. However, the reaction’s mechanism is not yet well understood. A major debate is whether the rate-determine step (RDS) for the generation of multi-carbon (C2+) products is C-C coupling or CO hydrogenation. This work describes an experimental analysis of the RDS by investigating pH dependency, the kinetic isotope effect and the effect of CO partial pressure in relation to the activity of C2+ products. The experimental data shows the activity of C2+ products to be invariant with either pH or deuteration of the electrolyte, and a CO reaction order of 0 at CO pressure of 1 bar. These results elucidate the RDS of C2+ products to be the dimerization of two adsorbed *CO over polycrystalline Cu. This study provides comprehensive kinetic experimental data and a theoretical basis for subsequent enhancement of the production of C2+ products.