The origin of room temperature ferromagneticlike behavior in ZnO-based diluted magnetic semiconductors is still an unclear topic. The present work concentrates on the appearance of RT magnetic moments in just mixed ZnO/Co3O4 mixtures without thermal treatment. In this study, it is shown that the magnetism seems to be related to surface reduction of the Co3O4 nanoparticles, in which, an antiferromagnetic Co3O4 nanoparticle core is surrounded by a CoO-like shell. This singular superficial magnetism has also been found in other mixtures with semiconductors such as TiO2 and insulators such as Al2O3.This work was supported by CYCYT MAT2007-66845-C02-01, CSIC 2006-50F0122, CSIC 2007-50I015 and by the MRSEC Program of the National Science Foundation under Award Number DMR-0212302.Peer reviewed