Solid oxide interfaces are at the forefront of solid-state science and materials research, exhibiting very appealing properties for new devices. This work describes the appearance of unexpected magnetic phenomena related to solid-state redox reaction, which might be the origin of the recently discovered magnetic signals in oxide multilayers and ceramic mixtures. The magnetic signal arises only when dissimilar oxide nanoparticles are simply mixed at room temperature, and it is not observed when nanoparticles are of the same chemical composition. Therefore, the phenomenon is ascribed to an interfacial solid-state reaction. Raman and X-ray absorption spectroscopies, scanning electron microscopy, and vibrating sample magnetometer have allowed us to identify the origin of the ferrimagnetic response. It is due to an electrochemical surface reduction of Co3+ in octahedral coordination to Co2+. So, Co-(oct)(2+)-O-Co-(tetra)(2+) interactions are produced. The reaction is driven by the different surface basicity of the oxides and has allowed obtaining different degrees of reaction, and concomitantly a proportionally strong magnetic response, when Co3O4 nanoparticles are mixed with SiO2, Al2O3, TiO2, and ZnO (from a larger to a smaller ferrimagnetic response). Also, a mechanism by which Co3O4 nanoparticles become ferrimagnetic is proposed.