The toxicity of metal oxide nanoparticles (NPs) has aroused great concern over the past few years. However, there still remains the question whether the toxicity of the metal oxide NPs originates from the released ions or the NPs themselves. In this study, the metal ion release of CuO, Fe2O3, ZnO, Co3O4, Cr2O3, and NiO NPs in aqueous media was investigated, and their contributions to the metal oxide NPs' inhibition on the bioluminescence of Photobacterium phosphoreum were studied. It was found that the ions release of the metal oxide NPs in aqueous media was complex, depending on both the dissolution and adsorption processes of the metal oxide NPs. The relationships between the metal oxide NPs' antibacterial effects and their released metal ions could be divided into three categories: (1) the ZnO NPs' antibacterial effect was due solely to the released Zn2+; (2) the CuO NPs' antibacterial effect originated from both the released Cu2+,and the CuO particles; and (3) the antibacterial effects of Fe2O3, Co3O4, Cr2O3, and NiO NPs were caused by the NPs themselves. Our findings suggest that the ions release and their contributions to the NPs' toxicity should be considered in the toxicity evaluations of the metal oxide NPs. (C) 2016 Elsevier B.V. All rights reserved.