A PEM fuel cell membrane electrode assembly (MEA) was characterized by dynamic electrochemical impedance spectroscopy (dEIS) before and after cycling in a single cell configuration. The cell was subjected to 100 cycles between 0.6 V and 1.5 V vs. RHE in N-2/5% H-2 and 80 degrees C and 100% RH. Initially, the impedance-plane plots contained first- and fourth-quadrant behavior, which is resulting from a reaction mechanism at the cathode involving adsorbed intermediates. After the cycling, the impedance spectra changed to display first-quadrant behavior only. This is suggested to be due to particle growth and possibly the formation of edges between agglomerated particles. The results show that dEIS is a sensitive technique to detect even very moderate changes in electrocatalyst structure. (C) 2013 Elsevier B.V. All rights reserved.