Effects of electrode substrate and catalytic coating on the electrochemical reduction of CO2 to ethylene at a three-phase interface have been investigated and the reduction scheme examined on the basis of the results of in situ Fourier transform infrared (FTIR) studies. The electrolysis of CO2 at a negative constant potential is carried out with copper-, silver-, Monel metal-, and stainless-mesh electrodes in a concentrated halide solution, and the generation of C2H4 is not observed at all except on a pure Cu electrode. However, the fixation of copper(I) halide onto the other three substrates leads to the formation of C2H4 with considerable current efficiencies, and it is demonstrated that copper(I) halide works as an active catalyst for the selective conversion of CO2 to C2H4 independently of the nature of substrate. The following scheme is proposed for the electrochemical reduction of CO2 to C2H4 based on the in situ FTIR spectra: CO2 is first reduced to carboxylic acid by one-electron reaction, the "side-on" type intermediate -CH2CO- is then formed by the protonation, C-O bond cleavages, and C-C bond formation of this acid, and it is finally converted to C2H4.