Hexagonal-phase NaYF4/NaGdF4 core/shell nanocrystals were synthesized and investigated by X-ray photoelectron spectroscopy (XPS) using tunable synchrotron radiation. Based on the ratio of the Y3+ 3d to Gd3+ 4d core level intensities at varying photoelectron kinetic energies, we conclude that Gd3+ resides predominantly at the surface of the nanocrystals, proving a core/shell structure. These nanocrystals show potential for use as contrast agents in magnetic resonance imaging (MRI) applications and optical imaging.