A solid film of Nation has been prepared on the inner wall of a quartz tube. After pulsed photoexcitation, time-resolved electron spin resonance (TR-ESR) spectra have been recorded in the temperature range of 290-15 K. The spectra exhibit strongly electron-spin polarized signals in full absorption arising from the excited triplet state of Nafion. The absence of any resolved spectral features and the unusual narrowing of the TR-ESR signal linewidth with decreasing temperature are discussed in terms of the spin Hamiltonian of an excited triplet state with dynamic dipole-dipole interactions between the two unpaired electron spins. The existence of a triplet-triplet energy transfer process is considered, and a prominent role of hydrogen bonds of the surrounding water network is suggested. Finally, the time evolutions of the TR-ESR signals have been analysed, and the corresponding decay parameters have been obtained at low temperatures. Our results provide insight on the role of water as a medium for the conductivity mechanism in Nation membranes. Copyright (C) 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.