Electrochemical CO₂ reduction rates are limited in aqueous-based electrolytes by the low solubility of CO₂. We recently demonstrated that organic solvent-based CO₂ Expanded Electrolytes (CXEs) can solubilize multi-molar amounts of CO₂ at moderate pressures while retaining sufficient supporting electrolyte to facilitate electrochemistry. Up to an order of magnitude enhancement in CO₂ reduction rates to CO was achieved on polycrystalline Au electrodes at faradaic efficiencies approaching 80%. Herein, we show that similar enhancements are observed on Cu catalysts as well, on the basis of enhanced current flow. On both systems, a maximum in CO₂ reduction rate was observed at ca. 5 M CO₂ concentration (i.e., 3.1 MPa head-space pressure) beyond which the rate decreases. To explain this counterintuitive phenomenon, we developed a detailed COMSOL-based mechanistic model of CO₂ reduction in organic electrolytes under elevated pressures on Au electrodes. This model incorporates the dissimilar variations of the key physicochemical properties (viz., CO₂ concentration, CO₂ diffusion rate, and solution polarity) with CO₂ pressure. We thereby demonstrate that the overall rate is limited by CO₂ concentrations at lower than optimum CO₂ pressures. At pressures higher than the optimum the rate is limited by both an attenuation of the first electron-transfer step and an increase in the ohmic resistance of the system. Excellent quantitative match between experimental and model-predicted rates lend credence to the proposed underlying mechanism of electrocatalytic CO₂ reduction in CXEs. These fundamental insights provide guidance for the rational design and scaleup of highly efficient CXE-based CO₂ reduction systems.