The toxicity of nanoparticles (NPs) depends on several factors including size, shape, surface properties and chemical nature of the NPs. The release of toxic ions due to the dissolution of NPs is another important factor. In addition, impurities or reaction products from synthesis procedures on the NP surfaces may contribute to the toxicity. Zinc oxide nanoparticles (ZnO NPs) are one of the unique NPs showing toxicity through all of these mentioned factors. In this study, we demonstrate that the treatment of the ZnO NPs with hydrogen peroxide (H2O2) alters the surface properties of the ZnO NPs by decomposing organic impurities remained from synthesis procedures. The changes on the surface chemistry and properties of the ZnO NPs influence their behavior in cell culture media and the NPs-cell interactions. Finally, a decrease in the cytotoxicity of H2O2 treated ZnO NPs is observed on HDF and A549 cells through the decrease of the membrane damage and oxidative stress. (C) 2014 Elsevier B.V. All rights reserved.