This paper proposes a hypothesis for explaining Pt/C particles coarsening inside the catalyst layers of a PEM fuel cell The hypothesis includes the two parts (1) due to capillary condensation a water bridge could be formed between two neighboring nano scale Pt/C particles at relative humidity under 100% when the surfaces of the Pt/C particles are hydrophilic, (2) the capillary force of the water bridge tends to pull together the Pt/C particles The relation is derived in this paper between the capillary force and the factors including the diameter of Pt/C particles relative humidity temperature distance between the two neighboring Pt/C particles and water contact angle A parametric study is per formed showing some details about water bridge formation Finally the stress level induced by the capillary force inside the Nafion thin film connecting with the Pt/C particles is calculated The result shows that the capillary force could be large enough to break apart the Nafion thin film facilitating the movement of Pt/C particles towards each other Crown Copyright (C) 2010 Published by Elsevier Ltd on behalf of Professor T Nejat Veziroglu All rights reserved