Characterizing the electronic structure of the titanium dioxide–aqueous interface is crucial for enhancing H2 fuel production efficiency in photoelectrochemical cells. Despite intense research on the nature of water interaction, it remains unclear whether water adsorbs dissociatively, associatively, or mixes at the TiO2 surface. Using liquid-jet photoelectron spectroscopy, we can access the electronic structure of the TiO2 nanoparticle–aqueous interface. One crucial aspect here is that the nanoparticles are fully dispersed in an aqueous solution, which allows aqueous ions to diffuse freely from the interface into the bulk solution. Our system thus mimics the conditions within photoelectrochemical cells. By measuring core-level photoelectron as well as resonant photoemission spectra at the oxygen 1s edge, we find that in acidic aqueous solutions, water adsorbs molecularly at the Ti sites. In contrast, water interacts dissociative at slightly basic pH. Variation of pH thus provides a means to control this interaction and amount of produced H2 fuel production.