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The through-plane thermal conductivity of artificially aged SGL Porous Transport Layers, PTLs, are measured and reported. NMR measurements of PTFE content and water/PTL contact angles are also reported. The PTLs were artificially aged in air rich water at 80 °C for 0, 200, 400, 600, 800, and 1000 h. For the dry samples, it was found that the through-plane thermal conductivity did not change significantly in response to the ageing treatment. For the samples containing water, the through-plane thermal conductivity increased by a factor of two to three and the more aged samples had the highest thermal conductivities. The through-plane thermal conductivity of dry PTLs is known to decrease with increasing PTFE content, which was also seen in this study. The chosen ageing procedure is known to wash PTFE from the PTL, and this verified using NMR. Water-PTL contact angle measurements also demonstrate that the PTL becomes less hydrophobic with ageing. This coincides with the increase in the through-plane thermal conductivity. Because the dry PTL retains its thermal conductivity while the PTFE content decreases we suggest that the PTFE remains in the fibre to fibre contact region and is removed predominantly elsewhere.