This thesis presents electronic-structure studies of water and aqueous salt solutions using liquid-jet soft-X-ray photoemission (PE) spectroscopy. Novel methods and detection schemes are employed to explore changes in electronic structure, ion-solvation, and nuclear dynamics under soft-X-ray irradiation. The initial focus is on the impact of atomic ions on water's electronic structure. Despite expectations, high salt concentrations show negligible effects on the electronic structure of liquid water, with an almost unchanged PE spectrum observed from neat water to 8 molar NaI concentration. This stability is attributed to efficient screening of ionic charges by polarizable water molecules. Additionally, new spectroscopy methods are introduced that utilize non-local autoionization processes following core-ionization, revealing energy and electron transfer between neighboring molecules. Two nonlocal decay mechanisms, intermolecular Coulombic decay (ICD) and electron-transfer mediated decay (ETMD), are discussed as general phenomena in weakly interacting systems. The thesis highlights proton nuclear dynamics initiated by soft-X-ray irradiation of the ammonium cation, NH4+(aq), where ICD probability correlates with hydrogen-bond strength. Notably, a complete proton transfer occurs within the ultrashort lifetime of the N 1s vacancy (~7 fs). The work also demonstrates the sensitivity of ETMD spectroscopy to local molecular arrangemen
