Thermal conductivity of the polymer electrolyte membrane fuel cell (PEMFC) components has achieved increased attention over the past decade. Despite the fact that the PEMFC itself (between the gas flow plates) is less than a millimetre in thickness, several C temperature differences can arise inside it during operation. These temperature differences mainly arise across the porous transport layers (PTL) often also referred to as gas diffusion layers (GDL). Several research efforts have led to a good understanding of the thermal conductivity of the PTL; in particular to how this property changes with compression, temperature, PTFE content, different fabrics, and water content. Far less attention has been given to the thermal conductivity of the much thinner layered micro porous layer (MPL) and in particular to the thermal conductivity of the transitional region between the PTL and the MPL. In this study we have used X-ray computer tomography (XCT), scanning electron microscopy (SEM), and thermal conductivity measurements to show that a MPL coated PTL is actually a three layered structure where the PTL is on one side, the MPL on the other, and a composite region consisting of the MPL as a matrix with the PTL fibres in the middle. We have shown that the thermal conductivity of the MPL-PTL-composite region is much larger than for the two others and that temperature differences inside this layer can be neglected compared to the regions where it is MPL-only and PTL-only. We have also shown that the MPL has a significantly lower thermal conductivity than the other two layers. In light of this research, the MPL of the commercial SGL should be integrated into the GDL in order to have lower temperature deviations in the PEMFC. A relevant literature review is included. Copyright (C) 2015, The Authors. Published by Elsevier Ltd on behalf of Hydrogen Energy Publications, LLC. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).