Increasing attention has been paid to magnetite nanoparticles (MNPs) due to their highly reductive reactivity toward environmental contaminants. However, there is little information related to the generation of reactive oxygen species (ROS) by MNPs, which in fact plays a vital role for the transformation of contaminants. In this paper, the degradation of 2-chlorobiphenyl (2-CB) by MNPs was investigated. The role of ROS generated by MNPs in this process was elucidated. The results demonstrated that hydroxyl radicals ((OH)-O-center dot) generated by MNPs at low pH could efficiently degrade 2-CB. The mechanism of the formation of (OH)-O-center dot by MNPs was divided into two steps: (i) the superoxide radical anion (O-2(center dot-)) mediated production of hydrogen peroxide (H2O2), and (ii) the reaction of formed H2O2 with Fe(II) dissolved from MNPs to produce (OH)-O-center dot through Fenton reaction. Comparison of the degradation products of 2-CB by MNPs with MNPs/ethanol and Fenton reagents further supported the involvement of (OH)-O-center dot in the degradation of 2-CB. The degradation efficiency of 2-CB by MNPs under acidic conditions was higher than that in alkaline solution. These findings provide a new insight into the understanding of reactivity of MNPs for the transformation of 2-CB and possibly other relevant environmental contaminants. (C) 2013 Elsevier B.V. All rights reserved.