BiOXs (X = Cl, Br, I) were prepared and their abilities to degrade microcystin-LR (MC-LR) under UV light were compared. They degraded MC-LR in the order of BiOBr > BiOCl > BiOI and the oxygen consumption was in the order of BiOBr > BiOI > BiOCl, which demonstrated that BiOBr exhibited the highest degradation rate and O-2 consumption. The significant differences in the degradation rate and net O-2 consumption between three photocatalytic systems containing BiOCl, BiOBr and BiOI are attributed to different valence band potentials (E-vb) of BiOXs. BiOCl (E-vb = 3.85 eV) oxidizes H2O to produce O-2 and this reaction is primary, rather than directly degrades MC-LR by positive valence band holes (h(vb)(+)). BiOI (E-vb = 2.66 eV), displays the slowest degradation rate due to its very low oxidative potential towards MC-LR. BiOBr is the most effective photocatalyst because its valence band potential (E-vb = 3.02 eV) is intermediate, which helps to avoid occurrence of side-reaction of H2O splitting to O-2. The most important results of our investigation demonstrate that h(vb)(+) can be tuned to optimize photocatalysis. (C) 2014 Elsevier B.V. All rights reserved.