High-resolution X-ray photoelectron spectroscopy is used to identify the mechanism of X-ray-induced Ag diffusion into Ge30Se70 chalcogenide glass thin films, which are prepared in situ to avoid oxygen contamination. From the analysis of Ge 3d, Se 3d, and Ag 3d core levels, and valence band spectra, changes in the electronic structure are determined as Ag diffuses gradually with increasing irradiation. The ternary phase based on Ge2Se6 units, which contains homopolar Ge-Ge bonds, forms when diffusion approaches equilibrium where Ag content similar to 30 at.%. The formation of a Se-rich composition is indicated in the near-surface region at the initial stage of the process, but the previously assumed Ag2Se phase is not detected.