Ab initio simulations on proton transport in water have been conducted. Using the simulation results together with the experimental data in the literature, currently existing hypotheses, including the one proposed by Agmon [N. Agmon, The Grotthuss mechanism, Chem. Phys. Lett. 244 (5-6) (1995) 456-462], have been examined. Based on the results of the simulations including charge distributions and the movement of the positive charge centers inside the protonated water clusters during the proton diffusion process, one mechanism is found to dominate proton transport in water. The high mobility of protons inside water is mainly due to the high diffusion rate of H5O2+ cations. The diffusion of H5O2+ cations is mainly induced by the thermal movement of water molecules in the second solvation shell of the H5O2+ cations and the Zundel polarization inside the cations. Furthermore, thermal effects play a dominant role during the transport process by affecting the reorientation of water molecules in the neighborhood of the second solvation shell of H5O2+ cations to induce the Zundel polarization and by providing the energy for the cleavage of the hydrogen bond between a water molecule and a newly formed H5O2+ cation. In addition, an external electrical field plays an important role in helping the water molecule reorient and lowering the Zundel polarization energy barrier. Because the weight fraction of H5O2+ cations among the protonated water clusters decreases as the temperature increases, the proposed mechanism is considered to play a dominant role only at temperatures below 672 K. (c) 2006 Elsevier B.V. All rights reserved.