Structural changes in a polymer electrolyte membrane (PEM) and a membrane electrode assembly (MEA) of different catalyst thickness under humidity cycle tests were investigated. Nafion211 membrane was used in this study. The catalyst layer thickness of MEAs was set to 2-10 mu m using catalyst paste of the same composition. Dimensional changes in the PEM and MEA due to swelling were measured, and dynamic mechanical analysis (DMA) of the membrane and MEA were carried out. Moreover, humidity cycle tests for the MEA were carried out according to the conditions specified in the FCCJ protocol, and structural changes in the MEA after the tests were examined. It was shown that the membranous structural change under humidity cycle tests tends to become small when the thickness of the catalyst layer is large. (C) 2014 Elsevier B.V. All rights reserved.