Hydration of neutral and cationic imidazole is studied by means of ab initio and
molecular dynamics calculations, and by photoelectron spectroscopy of the neutral species in a
liquid microjet. The calculations reveal the importance of long range solvent polarization and of
the difference between the structure of water molecules in the first shell around the neutral vs
cationic species for determining vertical and adiabatic ionization potentials. The calculated
vertical ionization potential of neutral imidazole of 8.06 eV agrees well with the value of 8.26 eV
obtained experimentally for an aqueous solution at pH 10.6.