Abstract We summarize our development of instrumentation for the study of liquids, aqueous solutions, and liquid/solid interfaces using resonant inelastic soft X-ray scattering (RIXS) and illustrate the value of the experimental approach with a few instructional examples. Using a high-transmission, high-resolution soft X-ray spectrometer, we are able to measure complete RIXS maps, i.e., record the soft X-ray emission intensity as a function of emission and absorption energy. As a first example, we show that a comparison of RIXS maps of “normal” and deuterated liquid and gas-phase water allows us to identify dissociation processes on the time scale of the oxygen 1 s core-hole lifetime. Similar dissociation effects are found for aqueous solutions of ammonia and amino acids. For the latter, the pH value has a strong influence on the nitrogen K emission spectra, which can thus be used to identify protonation and deprotonation processes in the solution. Finally, we review the investigation of the interface between liquid water and a CuIn(S,Se)2 thin-film solar cell absorber, demonstrating the power of the technique to study liquid–solid interfaces in real-world systems. Under X-ray irradiation, the formation of sulfate on the absorber surface can be found.