In this paper six High Temperature PEM (HTPEM) MEAs from two manufacturers have been tested. The MEAs are three Dapozol 77 from Danish Power Systems (DPS) with varying electrode composition and two Celtec P2100 and one Celtec P1000 from BASF. The break in process of the MEAs has been monitored using voltage measurements and impedance spectroscopy. The purpose of this study is twofold. One aim is to try and interpret the processes happening during break in. The other aim is to investigate whether the impedance spectra or the voltage profiles contain information that can be used to determine when an MEA has been broken in. To aid in the interpretation of the impedance spectra, equivalent circuit models are used. Three models are evaluated. The most detailed models produce the best fits but the most simple model is chosen, since it produces the most consistent results. The processes happening during break in cannot be determined with certainty but for the Celtec P MEAs the main changes seem to be related to improved electrode kinetics. Judging from the voltage and the fitted resistances, the Celtec P MEAs seem to have been broken in after 30 h. The Dapozol MEAs only undergo minor changes in impedance and voltage during the break in period. This may indicate that this MEA type can be used directly without the need for break in. Copyright (C) 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.