A new method for the hydroformylation of ethylene using a gas cell composed of (C2H4, CO, cathode H3PO4 aq. anode(Pt-black), H-2) has been examined at ca. 373 K and atmospheric pressure. Among many cathode electrocatalysts tested, H2PtCl6 showed the highest selectivity and high activity for the formation of C2H5CHO. However, H2PtCl6 did not catalyze the reaction at all under a gas mixture of C2H4, CO, and H-2. H2PtCl6 was specifically active for the electrocatalytic hydroformylation by separating H-2 from C2H4 and CO with a H3PO4-electrolyte membrane. The addition of Na3PO4 to the cathode enhanced the rate of C2H5CHO formation as well as the selectivity. The cell reaction does not require electrical energy input but takes place spontaneously and most efficiently under short-circuit conditions. The results from kinetic and electrochemical studies have suggested that the catalytic active species is the Pt2+-chloride supported on the host graphite. A stronger coordination of CO compared to those of C2H4 and hydride reduces both the hydrogenation and hydroformylation of C2H4. Therefore, a low partial pressure of CO and higher partial pressures of C2H4 and H-2 are recommended for the conversion of C2H4. The optimum temperature for the hydroformylation was 373 K. The results of transition response experiments have suggested a rapid reversible coordination of C2H4 but a slow conversion of the precursor of C2H5CHO on the Pt2+ sites. A tentative reaction mechanism assuming the formations of ethyl-carbonyl, ethyl-carbonyl hydride and acyl-carbonyl Pt complexes has been proposed on the bases of the results in this work.