CO2 electroreduction was examined on high-area metal (Fe, Ni, Cu, Pd) electrocatalysts supported on activated carbon fibers (ACF), which contain slit-shaped pores with widths on the order of 2 nm. These catalysts exhibited relatively high selectivity (ca. 67% total current efficiency for O-2 reduction on ACF/Ni) and high partial current density (up to 63 mA cm(-2) for CO2 reduction on ACF/Ni). Much smaller activities were observed for the same types of metal catalysts supported on nonactivated carbon fibers. The enhancement of the CO2 reduction selectivity with the microporous support is thought to involve a nanospace effect, which gives rise to high-pressure-like effects at ambient pressure. The CO2 reduction mechanism on Ni is proposed to involve an adsorbed CO2-CO2.- adduct, which is known to exist on the (110) surface.