In this contribution, we focus on the 1s O IE of bulk water. The gas-phase value is wellknown : 539.82 (±0.02) eV (value taken from Ref. 12). The question is then what is the magnitude of the shift of the 1s O level in the bulk relative to the gas-phase IE (∆IE). As in the valence domain, the IEs of the solvated neutral species are expected to red-shift relative to the gas phase due to the strong solvent stabilization of the resulting cationic state. The magnitude of the shift quantifies the overall effect of the solvent stabilization of the core-hole state, whereas the width of the peak gives a measure of the solvent's structural fluctuations.