Pt dissolution under potentiostatic and potential cycling conditions have been investigated in a 0.5 M H2SO4 solution at 298 K using a channel flow double electrode (CFDE) combined with EPMA and ICP-MS to clarify a dissolution mechanism of a Pt cathode catalyst in PEMFC. Under potentiostatic conditions, dissolved Pt ions are detected at a potential higher than 0.8 V with ICP-MS. The dissolution shows a maximum at E = 1.1 V and is suppressed by formation of 1-2.5 monolayers (ML) of Pt-O at E ≥ 1.2 V. Under potential cycling conditions, Pt dissolution is enhanced when the upper potential limit is higher than 0.8 V and the lower potential limit is less than 0.6 V, where the Pt-O is completely reduced. Dissolution is more enhanced as the upper potential limit shifts more positively. The CFDE study revealed that the dissolution is accelerated in the anodic scan under potential cycling even though more than 1 monolayer of Pt-O is formed, which is different from the potentiostatic conditions, and that the reductive dissolution of PtO2 occurs in the cathodic scan. A Pt dissolution mechanism is proposed in simulated PEMFC cathode conditions.