The interactions of H3O+ with water and model Nafion structures are studied using ab initio, density functional theory, and molecular dynamics simulations. In the gas phase, H3O+ is solvated by a well-defined first solvation shell with three water molecules, while an additional water molecule locates in the second shell. This same structure is-preferred in the liquid phase. In the absence of an electric field, proton transfer between H3O+ and a water molecule proceeds without barrier if the hydrogen bond length O-H...O is smaller than 2.7 Angstrom. When an electric field is applied in a direction opposite to that of the system dipole, the activation barrier for proton transport is significantly reduced. The Nafion side chain, ending in a sulfonic group, is folded in the gas phase, whereas in aqueous media, it becomes stretched, and proton transfer takes place from the acid group to the water molecules. Results from MD simulations indicate that the contact ion pair formed between the sulfonic acid anion -SO3-and the hydronium ion is very stable.