The current paper investigated the removal of the azo dye Orange II from water using advanced oxidation processes based on sulfate radicals. The cobalt oxide catalyst immobilized on graphene oxide (GO) can activate peroxymonosulfate (PMS) for the degradation of Orange II in water. The Co3O4/GO catalyst system was characterized via X-ray diffraction, Fourier transform infrared spectroscopy, Raman spectroscopy, scanning electron microscopy, and X-ray spectroscopy. Results showed that Co3O4 was distributed on GO. The Co3O4/GO catalyst system exhibited high activity in Orange II oxidation when the Co3O4/GO catalyst has an optimum Co3O4 loading. In addition, 100% decomposition could be achieved within 6 min with 0.2 mM Orange II, 0.1 g L-1 catalyst, and 2 mM PMS. Meanwhile, inductively coupled plasma analysis revealed that the leach of cobalt ions was low. The catalyst also exhibited stable performance after several rounds of regeneration. Several operational parameters, such as catalyst amount, oxidant amount, pH, temperature, and oxidation rate, affected the degradation of Orange II. (C) 2012 Elsevier B.V. All rights reserved.