The electronic structure and proton-transfer mechanism of CsHSO4 in phase I have been studied by the first-principles method. The calculated result shows the band gap is 6.03 eV, indicating that it is electronically insulating. Proton transfer paths and barriers have been investigated by the transition state theory. The calculated results indicate that the reorientations of the sulfate tetrahedron can take place very frequently, which is in accordance with the experimental observation. The proton-transfer mechanism for this material is then proposed. (C) 2004 Elsevier B.V. All rights reserved.