Thermal conductivities and compression of differently composed Polymer Electrolyte Membrane Fuel Cell (PEMFC) Catalyst Layers (CLs) were measured, both when dry and when containing liquid water. The results were compared using a 1-D thermal model. Dry CLs, and CLs with low water content, were found to have thermal conductivity values of 0.07-0.11 W K-1 m(-1) when compressed in the range of 5-15 bar compaction pressure. When adding water up to 70 mol of water per mole of sulphonic group, it was observed that the water only had an effect on the thermal conductivity with a water content significantly beyond the capacity of the ionomer. This means that water held by and between the catalyst particles contribute to the increase in thermal conductivity rather than water in the ionomer. The CLs were all found to compress irreversibly and to become incompressible above 10 bar compaction pressure. When considering wet porous transport layers (PTL) and moderately humidified CL, the PEMFC maximum internal temperature difference increased by 33% when compared to the commonly assumed CL thermal conductivities. Copyright (C) 2014, The Authors. Published by Elsevier Ltd on behalf of Hydrogen Energy Publications, LLC.