Multi-walled carbon nanotubes (MWCNTs) were used to reinforce Nafion resin to improve freeze/thaw durability of proton exchange membrane fuel cells (PEMFCs). Freeze/thaw cycling was conducted over 160 cycles for MWCNTs/Nafion membranes as well as conventional Nafion-112 membranes. The performance of the membranes was measured by recording polarization curves after every 40 freeze/thaw cycles. The fuel cells with the MWCNT-reinforced membrane showed better performance than the conventional Nafion-112 membrane after freeze/thaw cycling. H(2) permeability tested by the chronocoulometry method showed that the reinforced membrane, due to its improved mechanical properties and lower swelling, was able to tolerate the cyclical expansion and contraction caused by ice formation better than a Nafion-112 membrane of the same thickness. Scanning electron microscope tests revealed that the delamination between the catalyst layer and the membrane is greatly reduced for the reinforced membrane due to its improved dimensional stability. A 100-hour durability test under standard conditions showed good operational stability of the MWCNT-reinforced Nafion membrane. (C) 2011 The Electrochemical Society. [DOI: 10.1149/2.082112jes]