We recently reported the observation of room temperature ferromagnetism in mixtures of ZnO and Co3O4 despite the diamagnetic and antiferromagnetic character of these oxides, respectively. Here, we present a detailed study on the electronic structure of this material in order to account for the unexpected ferromagnetism. Electrostatic interactions between both oxides lead to a dispersion of Co3O4 particles over the surface of ZnO larger ones. As a consequence, the reduction Co+3 -> Co2+ at the particle surface takes place as evidenced by x-ray absorption spectroscopy measurements and optical spectroscopy. This reduction allows explaining the observed ferromagnetic signal within the well established theories of magnetism in oxides.