Efficient and selective electrosynthesis of propylene carbonate can be performed by the reaction of carbon dioxide with propylene oxide at copper electrodes. In this paper, we investigate this electrochemical reaction by using cyclic voltammetry, Fourier transform infrared spectroscopy and high-performance liquid chromatography in order to unravel details of the catalytic mechanism of the reaction. The combination of the results obtained by these different techniques allows the exclusion of different reduced forms of CO2, such as CO and (bi)carbonates, as possible carboxylation agents. Moreover, the results also indicate that electrochemical activation of the propylene oxide by ring opening is not the initial step for this reaction, as no product was detected when a current was not applied in presence of “activated propylene oxide” and CO2. Our results show that the reaction is initiated by the activation of CO2 to CO2.−, which then attacks the epoxide to form the cyclic carbonate. This work also gives evidence for the non-catalytic nature of the synthesis of the cyclic carbonate because its formation also occurs on other metals such as gold and platinum in the same range of applied currents. This result clearly indicates the potential of in situ electrochemical techniques in the mechanistic investigation of electrosynthesis reactions.