The methanation reactivity of surface carbon deposited from CO, on the oxygen-deficient magnetite was studied by the isothermal methanation reaction and the temperature programmed surface reaction (TPSR). In the methanation reaction with H-2 gas, in a closed system at 150-200 degrees C, active carbon, which was not observed by the TPSR measurements, was found. About 20% out of the deposited carbon was in the form of atomic carbon and readily converted into CH4 (5 min) above 150 degrees C with H-2, and about 80% at 300 degrees C. At 350 degrees C these atomic carbons were transformed into polymerized carbons, which was less reactive for methanation.