We investigate the reduction of CO2 to ethylene across buffered anolyte pH values 4 to 14 using a copper–phosphorus (Cu–P) electrocatalyst in a zero-gap membrane electrode assembly. Electrochemical CO2 reduction using alkaline electrolytes typically shows limited carbon efficiencies and single-pass efficiencies, while acidic conditions typically favor the hydrogen evolution reaction. Results from this work show that weakly phosphate-buffered acidic anolytes (pH 6) maximize ethylene production with a 73% FE at 300 mA cm–2 and 51% FE at 500 mA cm–2, including a 51% single-pass CO2 conversion efficiency for over 400 h of continuous operation. We propose a mechanism based on pH-dependent CO coverage that controls the selectivity at the *HCCOH intermediate. Low CO coverage at pH 6 favors hydroxide elimination to *CCH, yielding ethylene (98% of C2 products), while high coverage at pH 14 promotes hydrogenation to ethanol (44% of C2). The HER mechanism transitions from H2O-mediated at pH 14 to phosphate-mediated (H2PO4–/HPO42–) at weakly acidic pH, minimizing HER competition at pH 6. This mechanistic understanding enables controlled C2 product selectivity through manipulation of the CO coverage and local proton activity.