We study the vibrational dynamics of the bending mode at 1730 cm(-1) of proton hydration structures in Nafion membranes with polarization-resolved infrared (IR) pump-probe spectroscopy. The bending mode relaxes to an intermediate state with a time constant T-1 of 170 +/- 30 fs. Subsequently, the dissipated energy equilibrates with T-eq of 1.5 +/- 0.2 ps. The transient absorption signals show a long-living anisotropy, which indicates that for part of the excited proton hydration clusters the vibrational energy dissipation results in a local structural change, e.g. the breaking of a local hydrogen bond. This structural relaxation relaxes with a time constant of 38 +/- 4 ps. (C) 2016 Elsevier B.V. All rights reserved.