Recent nanomedical applications have again highlighted the significance of silica surface chemistry in solution. Here, we report in situ electronic structure measurements at the silica-aqueous interface as a function of pH for nanoparticles (NPs) of 7, 12, and 22 nm using a liquid microjet in combination with synchrotron radiation. The Si K-edge X-ray absorption near-edge spectroscopy (XANES) spectra reveal a change in shape of the Si 1s → t₂ (Si 2p-3s) absorption brought about by changes in the silanol protonation state at the interface of the NPs as a result of changes in solution pH. Our results are consistent with the number of silanol groups changing the protonation state being inversely correlated with the SiO₂ NP size. The importance of in situ studies is also demonstrated by comparing the XANES spectra of aqueous 7 nm SiO₂ with the same dehydrated sample in vacuum.