In this work we report the synthesis of zinc dioxide powder (ZnO2) with a polyhedral morphology, by oxidation of commercial zinc oxide (ZnO) using a 3% solution of hydrogen peroxide (H2O2). The effect of the reaction time on the ZnO to ZnO2 transformation was investigated. The oxidation process was carried out at room temperature and atmospheric pressure. The reaction times were 3, 5, 7.5, 10, 20, 40, 60, 120, 180, 480 and 1020 min. The generated samples were characterized by Raman micro-spectroscopy (R mu S), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), differential scanning calorimetry analysis (DSC), and thermogravimetric analysis (TG). The results indicate that the preparation method is adequate for the generation of pure ZnO2 as well as of ZnO-ZnO2 mixtures. These materials could find applications in photocatalysis and oxygen resources.