The stability of platinum in proton exchange membrane fuel cell (PEMFC) electrodes has been investigated by determining the dissolution of platinum from a thin platinum film deposited on a gold substrate in 1 M HClO4 at different temperatures ranging between 40 and 80°C and potentials between 0.85 and 1.4 V vs reversible hydrogen electrode (RHE). The loss of Pt during the dissolution process is monitored in situ with the highly sensitive quartz crystal microbalance. By combining the microbalance with the electrochemical cell, the dynamic change of the electrode weight can be observed in situ. It is shown that first an oxide layer is formed, which can be dissolved depending on temperature and potential. The dissolution rate is found to be strongly dependent on the potential and temperature. At a potential close to 1.15 V vs RHE, the dissolution rate becomes saturated at 80°C due to the protection of a passivating surface platinum oxide layer at high potential. At 80°C, the dissolution rate starts to diminish when the potential is higher than 1.15 V.