The electrochemical reduction of carbon dioxide in 0.1 M KHCO3 + H2O was investigated on a Pd membrane surface to which deuterium was supplied continuously from the back face. Isotope distributions of hydrogen and formic acid were measured. In comparison with simple hydrogen evolution under an N-2 atmosphere, the formation of HD was suppressed. This suggests the existence of hydrogen-rich and deuterium-rich regions on the electrode surface and is attributable to poisoning by the adspecies. More than 95% of the formic acid was HCOO-, i.e. it was formed by the incorporation of hydrogen from H2O. Considering that no formic acid was formed in the absence of hydrogen supplied from the back face, the surface hydrogen converted to formic acid is probably unstable, and is generated only on the hydrogen "flowing-out" surface. The effect of thiourea was investigated for poisoning or promotion of hydrogen-related processes and CO2 reduction. The mechanism of the electrochemical reduction of CO2 is discussed in relation to the role of surface and subsurface hydrogen.