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X-Ray photoelectron spectroscopy has been extended to colloidal nanoparticles in aqueous solution using a liquid microjet in combination with synchrotron radiation, which allowed for depth-dependent measurements. Two distinct electronic structures are evident in the Si 2p photoelectron spectrum of 7 nm SiO(2)-nanoparticles at pH 10. A core-shell model is proposed where only the outermost layer of SiO(2) nanoparticles, which is mainly composed of deprotonated silanol groups,Si-O(-), interacts with the solution. The core of the nanoparticles is not affected by the solvation process and retains the same electronic structure as measured in vacuum. Future opportunities of this new experiment are also highlighted.