In this work the thermal limitations of high current density proton exchange membrane water electrolysis are investigated by the use of a one dimensional model. The model encompasses in-cell heat transport from the membrane electrode assembly to the flow field channels. It is validated by in-situ temperature measurements using thin bare wire thermocouples integrated into the membrane electrode assemblies based on Nafion 117 membranes in a 5 cm(2) cell setup. Heat conductivities of the porous transport layers, titanium sinter metal and carbon paper, between membrane electrode assembly and flow fields are measured in the relevant operating temperature range of 40 degrees C - 90 degrees C for application in the model. Additionally, high current density experiments up to 25 A/cm(2) are conducted with Nafion 117, Nafion (R) 212 and Nafion XL based membrane electrode assemblies. Experimental results are in agreement with the heat transport model. It is shown that for anode-only water circulation, water flows around 25 ml/(min cm(2)) are necessary for an effective heat removal in steady state operation at 10 A/cm(2), 80 degrees C water inlet temperature and 90 degrees C maximum membrane electrode assembly temperature. The measured cell voltage at this current density is 2,05 V which corresponds to a cell efficiency of 61 % based on lower heating value. Operation at these high current densities results in three to ten-fold higher power density compared to current state of the art proton exchange membrane water electrolysers. This would drastically lower the material usage and the capital expenditures for the electrolysis cell stack. (C) 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.