Polymer electrolyte fuel cells (PEFC) are expected as an alternate energy conversion source for automobiles and stationary power supplies. However, degradation of the cathode catalyst of platinum is one of the problems for the commercialization of PEFCs. It is well known that platinum catalyst at the cathode of PEFC dissolves into the polymer electrolyte, and then the dissolved platinum ions are reduced by hydrogen gas leaked from the anode. As a result, in polymer electrolytes after long-term operation, a line of deposited platinum particles, the so-called platinum band, is formed. In this study, dissolution of platinum has been investigated in 0.5M H 2SO 4 solution using electrochemical quartz crystal microbalance (EQCM) and ICP-MS. In potentiostatic conditions, platinum dissolved at the highest rate at 1.0 V (vs. SHE). Before 1.0 V, the dissolution started from around 0.80 V and increased with the applied potential. The dissolution was inhibited after 1.0 V by passivation of platinum surface due probably to formation of PtO. Under the potential cycle condition (cyclic voltammetry), the dissolution was enhanced, especially when employed the upper and lower limits of potential cycles where formation and reduction of PtO occur. On the basis of the results, dissolution mechanism of platinum will be discussed.