In this study, the synergistic catalytic mechanism of Co3O4 and graphene oxide (GO) nanocomposite in the heterogeneous activation of peroxymonosulfate (PMS) to generate sulfate radicals was studied. The activation of Co3O4/GO with different Co3O4 loadings in the catalyst [bare graphene oxide (GO), 20% Co3O4/GO, 30% Co3O4/GO, 50% Co3O4/GO, 70% Co3O4/GO, 90% Co3O4/GO, 95% Co3O4/GO, and pure Co3O4] were tested though the degradation of Orange II in water. The results show that the highest performance achieved using the Co3O4/GO catalyst was based on a synergistic catalysis between Co3O4 and GO. Furthermore, a proportional relation exists between Co3O4 and the catalyst. The highest catalytic activity is observed when the Co3O4 loading was about 50% in the catalyst. Co-OH complexes form on the surface of the GO sheet through the direct interaction of Co species with nearby hydroxyl groups or through the dissociation of H2O with Co2+. The formation is proposed to facilitate the heterogeneous activation of PMS. However, a high Co3O4 loading on the GO surface prevents the formation of Co-OH complexes, which is important in the catalytic reaction. (C) 2013 Elsevier B.V. All rights reserved.