The adsorption isotherm and the enthalpy of adsorption of CO₂ on oxides of oxygen-deficient magnetite have been studied by adsorption techniques. Oxygen-deficient magnetite was prepared by flowing H₂ gas through magnetite powder at 300 °C. Adsorption of CO₂ onto oxygen-deficient magnetite was studied in the temperature range 150–300 °C. We found that the adsorption can be expressed by the Langmuir dissociative isotherm for three fragments: one carbon atom and two oxygen ions. Deposition of carbon after the adsorption reaction suggests that reduction of surface carbon seems to be involved in the adsorption reaction. The high reactivity for the reduction of CO₂ to carbon is considered to come from such a reactive site where an electron is readily donated to the carbon of the CO₂ molecule and the oxygen in the CO₂ molecule is readily incorporated into a lattice point in the form of O^2–. Electron hopping between the Fe²⁺ and Fe³⁺ ions in the spinel structure of the magnetite would facilitate the donation of an electron at the adsorption site. The distorted spinel structure of the surface of the H₂-reduced magnetite, where the oxygen site is defected, would facilitate the incorporation of the oxygen of CO₂.