The effect of H2O2 on the Pt dissolution in 0.5 mol dm(-3) H2SO4 was investigated using an electrochemical quartz crystal microbalance (EQCM). For the potential cycling at 50 mV s(-1) the Pt weight irreversibly decreases in a N-2 atmosphere with H2O2, while only a negligible Pt weight-loss is observed in the N-2 and O-2 atmospheres without H2O2. The EQCM data measured by the potential step showed that the Pt dissolution in the presence of H2O2 depends on the electrode potential and the H2O2 concentration. For the stationary electrolysis, the Pt dissolution occurs at 0.61-1.06 and 1.06-1.36 V vs. RHE. It should be noted that the Pt dissolution phenomenon in the presence of H2O2 is also affected by the potential scanning time. Based on these results, H2O2 is considered not only to contribute to the formation of Pt-oxide causing the cathodic Pt dissolution, but also to participate in the anodic Pt dissolution and the chemical Pt dissolution. Copyright (C) 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.