In this communication, we show that K-edge X-ray Absorption Spectroscopy (XAS) is a sensitive probe of e - (aq) 's solvation environment and provides direct evidence for σ * OH occupation by the excess electron. In particular, the intensity of transitions between water O1s and valence orbitals of the excess electron in its solvent cavity is sensitive to the size of the cavity and how much the valence states overlap with the 4a1 and 2b2 antibonding orbitals responsible for the pre-edge and main edge features of water's K-edge XAS. Within a simple one-electron molecular orbital picture, one would predict that K-edge XAS transitions to e - (aq) 's half-filled valence orbital appear at roughly 3.7 eV (e - (aq) 's vertical binding energy 36 ) below water's O1s vertical binding energy (538 eV 37 ), i.e. to the red of water's pre-edge peak at 535 eV. Recently, Loh et al measured the timeresolved XAS of liquid water following strong-field ionization. Although the focus of their study was on the formation and reaction of H2O + , they indeed noted an increased absorption in the low-energy tail of water's pre-edge feature that was tentatively assigned to changes in the hydrogen bond network around e - (aq) . Using a quantum mechanics/molecular mechanics description, we simulated the XAS of e - (aq) and liquid water at the time-dependent density functional theory (TDDFT) level (see SI for details), shown in Fig. . We find the presence of the excess electron leads to an enhancement of the low-energy tail of the pre-edge feature, in agreement with experiment. Considering the stick spectra (thin vertical lines), we find the enhancement arises from new transitions centered around 533 eV that have oscillator strengths about half the magnitude of the pre-edge transitions, which start above ∼534 eV.