The mechanism of the gas-phase reaction UF6 + H2O -> UOF4 + 2HF is explored using relativistic density functional theory calculations. Initially, H2O coordinates with UF6 to form a 1: 1 complex UF6 center dot H2O. Over an activation energy barrier of about 19 kcal/mol, H2O transfers a H atom to a nearby ligand F, resulting in UF5OH + HF. The eliminated HF or another H2O molecule may form a hydrogen bond with UF5OH. Starting from UF5OH, the second HF elimination results in UOF4. If UF5OH is in the isolated form, UF5OH -> UOF4 + HF takes place over a barrier of 24 kcal/mol. If UF5OH is hydrogen-bonded with H2O or HF, the conversion barrier is less than 10 kcal/mol. Once formed, the unstable UOF4 tends to associate with additional ligands and hydrogen-bonding donors. The calculated binding energies indicate the significance of such interactions, which may have profound impact on further HF eliminating reactions. The IR spectra features can be used to indicate the formation and interaction type of the intermediates and products.