The electron-donating property of Na metal-doped MgO (Na/MgO) was studied by the ESR technique. The spin concentration of(CO)(n)(x-) ((CO)(2)(-) and/or (CO)(6)(3-)) radicals formed from adsorbed CO on a Na/MgO surface was much higher than that of the surface paramagnetic center formed by an electron released from sodium metal and trapped in an anion vacancy. The ESR signal intensity of the trapped electron paramagnetic center was not changed by the CO adsorption treatment, and so the trapped electron was not transferred to the adsorbate. However, the surface one-electron-donating property was increased dramatically by the trapped electron, apparently due to an inductive effect.