The mechanism of proton transport in a trifluoromethane sulfonic acid monohydrate solid is investigated using ab initio molecular dynamics. This system provides a model for proton transfer in minimally hydrated sulfonic acid based polymer electrolyte membranes, materials of technological importance in fuel cells. As a mechanism of proton transport, these simulations identify a defect involving the formation of an H5O2+ ion and the re-organization of neighboring sulfonate groups, which share a proton between the oxygen atoms of the anionic sites. The energy of formation of this defect (0.3 eV) agrees with the experimentally determined activation energy for proton transport in minimally hydrated Nafion(R) of 0.36 eV. (C) 2002 Elsevier Science B.V. All rights reserved.