Electron carriers were generated in refractory oxide 12CaO (.) 7Al(2)O(3) (C12A7) through a thermal treatment in a reducing CO/CO2 atmosphere, and the reduction process was thermodynamically analyzed. Electrical-conductive C12A7, which had an electron concentration of similar to 8 x 10(19) cm(-3) and an electrical conductivity of similar to 4 S/cm at 300 K, was obtained by extracting free O2- ions in sub-nanometer-sized cages in the C12A7 lattice and leaving electrons behind. The enthalpy for this substitutive reaction is +425 kJ/mol, suggesting that stoichiometric C12A7 is more stable in air than the C12A7 electride.