The vehicle that use a Polymer Electrolyte Membrane Fuel Cell (PEMFC) as a power source frequently experiences start up and shut down Membrane Electrode Assembly (MEA) degradation by wet/dry gas repetition was studied for vehicle start up and shut down The time of the wet/dry equilibrium state on the PEMFC was measured with High Frequency Resistance (HFR) The gas injection time was 20 mm and 5 mm for dry gas and wet gas respectively An experiment was carried out using electrochemical methods and a cross section of the MEA was visualized with a Field Emission Scanning Electron Microscope (FESEM) After 1200 wet/dry cycles, the performance of the cell decreased by 45 7% to its current density of 800 mA/cm(2) Ohmic and charge transfer resistances of the cell increased in the Electrochemical Impedance Spectroscopy (EIS) The crossover current of hydrogen also increased in the linear sweep voltammetry (LSV) The reduction of the electrochemical active surface area (ECSA) was confirmed through cyclic voltammetry (CV) The interface among the membrane catalyst layer, and gas diffusion layer was separated and significantly deteriorated compared with fresh MEA (C) 2010 Published by Elsevier Ltd on behalf of Professor T Nejat Veziroglu