Atomistic simulations have been carried to investigate electric field induced transport of hydronium ions in a sulfonated poly(styrene-co-divinylbenzene) membrane. In order to provide a good description of this cross-linked material, a methodology has been explicitly designed to construct a reliable model of the hydrated membrane. This model has been used to carry out molecular dynamics simulations in presence of electric fields, which varied from 0.001 to 3.0 V3 nm-1. Results show that the electric field affects the structure of the membrane producing both a redistribution of the unoccupied volume, which modifies the heterogeneity of the resin, and a rearrangement of the negatively charged sulfonate groups, which undergo a systematic alignment along the electric field direction. As was expected, the mobility of hydronium ions is enhanced with the strength of the electric field. Moreover, the electric field induces a significant rearrangement of the sulfonate groups, which is evidenced by the alignment of theC-S bonds along the direction of the field. The membrane has been found to behave as a spring, in which the force exerted by the electric field acts in opposite sense to the force exerted by the internal structure of the cross-linked material.