High-level quantum mechanical calculations have been performed systematically for hydrated proton clusters to understand the mechanism of proton transfer in water. Recently developed multi-coefficient correlated quantum mechanical methods, in particular, MCCM-UT-MP4SDQ, and MCCM-UT-CCSD, were used to calculate energies. Structures and frequencies were calculated at the MP2 level using the TZ2P and cc-pVTZ basis sets. The hydration enthalpies from the MCCM methods agree very well with experimental values. The conversion between H9O4+(I) and H9O4+(II) clusters was suggested to be important for aqueous proton transfer. (C) 2002 Elsevier Science B.V. All rights reserved.