Despite its fundamental significance, the 1s O IE of pure water is not known with sufficient precision. The reported experimental values of the shift of the bulk IE relative to the gas-phase value range from -1.3 to -2.8 eV. The main uncertainty in the experimental determination is due to the interface potential (or the difference between the workfunction of the liquid and the analyzer), which distorts the positions of both the gas-phase and bulk peaks; additional uncertainties may be due to the presence of electrolytes.