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The identification of the rate-determining step (RDS) in the electrochemical CO/CO₂ reduction to multi-carbon (C₂₊) products has been complicated by the deficiency of rigorous reaction kinetic data. This work describes an experimental analysis of the key reaction steps by exploring the effect of CO partial pressure on the activity of C₂₊ products. With the aid of a flow electrolyzer integrated with a gas diffusion electrode, the distinct reaction orders of CO and reaction mechanisms in forming different C₂₊ products were determined. Specifically, *CO dimerization is identified as the RDS for ethylene and ethanol production, as evidenced by the gradual transition of measured CO reaction order from second to zero as CO partial pressure increases from 0.05 to 1 atm. The formation of n-propanol is suggested to proceed via the *CO trimerization mechanism. The acetate generation mechanism might involve a critical step of …