Cobalt oxide catalysts immobilized on various oxides (MgO, ZnO, Al2O3, ZrO2, P25, SBA-15) were prepared for degradation of organic dyes in dilute solutions via a sulfate radical approach. Their efficiency in activation of peroxymonosulfate (PMS) was investigated for the degradation of methylene blue (MB). Among the catalysts employed, the Co/MgO catalyst was found the most active. The complete degradation of MB occurred in <7 min when the Co/MgO catalyst with an optimum Co3O4 loading of 5 wt% was used. The performance of the Co/MgO catalyst is found better than both the homogeneous cobalt ions and heterogeneous Co3O4 catalyst. XPS analysis indicates that the surface of the MgO support is extensively covered by the hydroxyl groups. Hence, it is suggested that the alkaline MgO support plays several important roles in (i) dispersing the cobalt oxide nanoparticles well, (ii) minimizing the leaching of cobalt ions into the liquid phase, and (iii) facilitating the formation of surface Co-OH complex which is a critical step for PMS activation. Besides MB, other organic dyes such as orange II and malachite green, can also be degraded within a few minutes using the Co/MgO catalyst. It is believed that the highly efficient and environmentally benign Co/MgO catalyst developed in this work can be widely applied in advanced oxidation technologies towards degradation of organic pollutants. (C) 2009 Elsevier B.V. All rights reserved.