Cost-effective cobalt oxides were synthesized at mild condition as alternatives to platinum catalyst for oxygen reduction in microbial fuel cells (MFCs). The catalysts were prepared by heating Co(NO3)(2) above its decomposition temperature together with carbon black (CB). At a catalyst loading of 5 mg/cm(2), cobalt oxide cathodes with a Co(NO3)(2) to CB ratio of 0.732:1 produced a maximum power density of 1220 mW/m(2), which was slightly lower than that of Pt/C cathodes (1360 mW/m(2)). Further increase in the loading of cobalt oxides resulted in an increase in performance. The maximum power densities produced by cobalt oxide cathodes increased to 1400 mW/m(2) at a catalyst loading of 20 mg/cm(2) and 1540 mW/m(2) at 30 mg/cm(2), which outperformed the Pt/C control. The durability of cobalt oxide catalysts was comparable to that of Pt/C in single-chamber MFCs with biological contamination. The costs of cobalt oxide catalysts were much lower than that of Pt/C catalyst, even when the loading of cobalt oxides was several times higher than Pt/C catalyst. These findings show that the facilely synthesized cobalt oxides were cost-effective catalysts for oxygen reduction in air-cathode MFCs.