Imide crosslinked perfluorinated proton exchange membranes (PEMs) were prepared by treatment of perfluorinated sulfonyl fluoride membranes with ammonia and trimethylamine gases to improve the mechanical properties at an elevated temperature. Tensile testing and infrared (IR) spectroscopy results indicated that imide crosslinks were successfully introduced into the perfluorinated PEMs. The imide crosslink density could be controlled by the gas treatment conditions. The imide crosslinked PEMs showed high mechanical properties and heat stability, i.e. far less creep elongation at elevated temperature than non-crosslinked PEMs. The imide crosslinked PEMs also exhibited a slight decrease in conductivity and cell performance due to a slightly lower ion exchange capacity than that of non-crosslinked PEMs. The introduction of imide crosslinking in PEMs is therefore a method for achieving improved mechanical properties and thermal stability with a minimum decrease in fuel cell performance. (C) 2016 The Electrochemical Society. All rights reserved.