We shed light on the mechanism and rate-determining steps of the electrochemical carboxylation of acetophenone as a function of CO2 concentration by using a robust finite element analysis model that incorporates each reaction step. Specifically, we show that the first electrochemical reduction of acetophenone is followed by the homogeneous chemical addition of CO2. The electrochemical reduction of the acetophenone-CO2 adduct is more facile than that of acetophenone, resulting in an Electrochemical-Chemical-Electrochemical (ECE) reaction pathway that appears as a single voltammetric wave. These modeling results provide new fundamental insights into the complex microenvironment in CO2-rich media that produces an optimum electrochemical carboxylation rate as a function of CO2 pressure.