An ab initio path integral molecular dynamics simulation has been performed to study the O-16 and O-18 isotopomers for a water molecule and a hydronium ion at temperature 300 K. The average O-H bond length of (H2O)-O-18 molecule is slightly shorter than that of (H2O)-O-16 molecule, while that of (H3O+)-O-18 is slightly longer than that of (H3O+)-O-16. For hydronium ions, the Walden inversion of (H3O+)-O-18, as well as (D3O+)-O-16, is found to be more restrained than that of (H3O+)-O-16. The isotope effect in the electronic structure and thermochemical properties for these isotopomers are also shown. © 2005 Elsevier B.V. All rights reserved.