Hydrogen fuel cell performance of sulfonated diels alder poly(phenylene) (SDAPP) with IECs >= 1.8 meq g(-1) is comparable to Nafion 212 under fully humidified conditions at 80 degrees C. However, as relative humidity is reduced, performance loss is substantial for SDAPP when compared to Nafion 212. This loss can be attributed to the large drop in proton conductivity in SDAPP as relative humidity is reduced; the proton conductivity of SDAPP with an IEC of 2.3 meq g(-1) dropped from 0.117 S cm(-1) to 0.001 S cm(-1) as the relative humidity was reduced from 100% to 25% at 80 degrees C. Methanol fuel cell experiments using 3M methanol result in a 60 mV performance improvement at 25 mA cm(-2) when using SDAPP with an IEC of 1.2 meq g(-1) instead of Nafion 212. This improvement is due to lower methanol permeability of SDAPP (1.4 meq g(-1)) over Nafion 212, with SDAPP films having methanol permeabilities less than 25% of Nafion 212. (C) 2009 Elsevier B.V. All rights reserved.