Display omitted ▶ Start-up/shut-down of a PEFC was simulated by exchanging H2 and air at the anode. ▶ Dye film was used to visualize pO2 at the anode during the H2/air exchange. ▶ Gas exchange rate was lowered by protons produced by carbon corrosion at the cathode. ▶ After degradation, oxygen was consumed mainly in the middle of the MEA. ▶ STEM images of the catalyst layer showed holes near the inlet and outlet of the cell. During start-up/shut-down processes of a polymer electrolyte fuel cell, platinum particles are lost from the catalyst layer at the cathode due to corrosion of the carbon supports. We simulated the start-up/shut-down cycle by exchanging gases at the anode between hydrogen and air. During the gas exchange, the distribution of oxygen partial pressures at the anode was visualized by our real-time/space visualization system, which clearly showed the location of H2- and O2-rich areas along the gas-flow channel from the inlet to the outlet. The gas exchange rate was found to be much slower than that predicted from the simple replacement and to be correlated to the proton transfer derived from carbon corrosion of the cathode catalyst layer. By the visualization results, it was found that the shut-down process gives more serious effect than the start-up process. After the degradation, the oxygen partial pressure at the cathode was visualized during the cell operation. Oxygen was consumed mainly in the middle of the MEA because the MEA was degraded mainly near the inlet and outlet of reactant gases in the cell.