The microporous layer (MPL) and the gas diffusion layer (GDL) in a polymer electrolyte membrane (PEM) fuel cell assembly are often treated as separate layers in the literature. However, there exists a considerable third region where the two different materials merge in the coating process. This region has properties that differ from either of the materials that it consists of. Through-plane thermal conductivity and thickness variation under different compaction pressures were measured for such a composite region of two different commercial GDLs, Freudenberg H1410 and Toray Paper TGP-H-030, each treated with a custom-made MPL ink. Thermal conductivity at 15 bar compaction pressure for untreated Freudenberg H1410 GDL is 0.124 +/- 0.009 W K-1 m(-1) and for the custom-MPL-coated Freudenberg H1410 materials it was increased by the treatment to 0.141 +/- 0.004 W K-1 m(-1) and 0.145 +/- 0.004 W K-1 m(-1) for 9.9 wt% and 11.9 wt% ink, respectively. For Toray paper TGP-H-030 the thermal conductivity at 15 bar compaction pressure for GDL only is 0.449 +/- 0.009 W K-1 m(-1) and for the custom-MPL-coated Toray TGP-H-030 materials it was decreased by the treatment to 0.39 +/- 0.05 W K-1 m(-1) and 0.39 +/- 0.00 W K-1 m(-1) for 9.9 wt% and 11.9 wt% ink, respectively. (C) 2018 The Electrochemical Society.