A solid film of Nafion 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 Nafion membranes.