The cathodic reduction and the optical properties of the peroxo-complexes [LCo-O2-CoL]+4 with L = en (ethylenediamine) and L = trien (triethylenetetramine) have been studied. It was found that four electrons are involved in the reduction of the peroxo-complexes. The diffusion coefficients of the above complexes were calculated to be 7.0 × 10-6 and 8.8 × 10-6 cm2 s-1 respectively. The redox potentials are -0.57 V and -0.50 V (vs. s.c.e.) respectively. These complexes are unsuitable as catalysts for the cathodic reduction of oxygen because they have lower redox potentials than that of oxygen. A different behaviour was found in the cobalt-ethylenediamine system when the ratio [en]:[Co(II)] = 1. The redox potential of the complex formed in this case is -0.17 V and its optical spectrum showed two absorption bands (λmax at 350 and 310 nm.) Therefore, it is suggested that a Co(III) en complex is formed, one which was found to be a suitable homogeneous catalyst for the cathodic reduction of oxygen.