Herein, we have developed an effective strategy to immobilize spinet cobalt oxide (Co3O4) particles of nanometer and subnanometer sizes on carbon sphere support via impregnation and deposition precipitation methods. With carbon spheres support, very small Co3O4 nanoparticles of around 1-2 nm are uniformly distributed on the surface. Based on XPS analysis, the surface cobalt species are found coordinated with hydroxyl groups from carbon spheres. Among the various catalysts investigated, the Co/C-300 nanocatalyst with 1-2 nm cobalt oxide clusters exhibits the highest activity with over 99% of methyl orange degraded in less than 10 min of reaction. The higher activity of Co/C-300 nanocatalyst was attributed to small size of Co3O4 nanoparticles with more surface sites exposed for dye degradation reaction. The Co/C-300 nanocatalyst is reasonably stable and could be recycled for at least four runs without losing activity substantially. (C) 2015 Elsevier B.V. All rights reserved.