The reversible oxygen-binding properties of the polyethyleneimine-cobalt (PEI-Co) complex and the effect of the PEI-Co complex on the reduction current of oxygen were studied. The color of the PEI-Co complex in the electrolyte solution changed on exposure to oxygen atmosphere

which was monitored by UV-visible absorption spectrometry．The UV-visible absorption spectra of the PEI-Co complex displayed a new adsorption band at 310 nm

with an isosbestic point at 280 nm．The spectral change was attributed to the reversible oxygen-adduct formation of the PEI-Co complex even in the electrolyte solution．The oxygen-binding equilibrium curve of the PEI-Co complex obeyed a Langmuir isotherm

to give the high oxygen-binding affinity (p50=0.667 kPa)．And the apparent dissociation rate constant of oxygen from the PEI-Co complex (kd=1.1×105 s-1) was also high．The aqueous solution of the PEI-Co complex that had rapid and reversible oxygen-binding properties functioned as an oxygen-enriching medium for oxygen electrode to enhance the diffusion-controlled current for the oxygen reduction．Based on the rapid release of oxygen from the PEI-Co complex

a high current was obtained for the reduction of oxygen in the presence of the PEI-Co complex

and the current increased with the increase of the concentration of the PEI-Co complex and the oxygen concentration in the atmosphere．The reduction current reached a saturated value near [ethyleneimine unit]／[Co]=5

which suggested the structure of a six-coordinate μ-dioxo dinuclear complex [N5CoⅢ-O2-CoⅢN5]．A new type of oxygen-diffusion electrode for metal／air batteries and fuel cells is proposed using the oxygen-enriching material immobilized at the electrode surface．