ZnO nanomaterials exhibit prominent antibacterial activity even without UV illumination. Their antibacterial activity was attributed to different mechanisms, such as production of reactive oxygen species (ROS) and/or release of zinc ions in solution. Here we examined the relationship between the properties of eight different ZnO nanoparticles, their production of ROS, zinc ion release and antibacterial activity. We found that while the particles exhibited differences in antibacterial activity, there was no correlation with ROS production and zinc ion release. However, ROS production was inversely correlated with the concentration of nonradiative defects indicating that the recombination losses of photogenerated carriers can negatively affect ROS production. (C) 2013 Elsevier B.V. All rights reserved.