Photoelectron spectroscopy combined with the liquid microjet technique enables 
the direct probing of the electronic structure of aqueous solutions. We report 
measured and calculated lowest vertical electron binding energies of aqueous 
alkali cations and halide anions. In some cases, ejection from deeper 
electronic levels of the solute could be observed. Electron binding energies 
of a given aqueous ion are found to be independent of the counter ion and the 
salt concentration. The experimental results are complemented by ab initio 
calculations, at the MP2 and CCSD(T) level, of the ionization energies of 
these prototype ions in the aqueous phase. The solvent effect was accounted 
for in the electronic structure calculations in two ways. An explicit inclusion
 of discrete water molecules using a set of snapshots from equilibrium 
classical molecular dynamics simulations, and a fractional charge 
representation of solvent molecules gives good results for halide ions. The 
electron binding energies of alkali cations computed with this approach tend to
 be overestimated. On the other hand, the polarizable continuum model, which 
strictly provides adiabatic binding energies, performs well for the alkali 
cations but fails for the halides. Photon energies in the experiment were in 
the EUV region (typically 100 eV) for which the technique is probing the top 
layers of the liquid sample. Hence the reported energies of aqueous ions are 
closely connected with both structures and chemical reactivity at the liquid 
interface, e.g., in atmospheric aerosol particles, as well as fundamental bulk 
solvation properties.