[en] Elastic recoil detection (ERD) analysis of GeO_x thin films was carried out to understand the mechanism of phase separation. Stoichiometry of the film was monitored on-line at various fluences of 100 MeV Au ions. Although significant electronic sputtering of GeO_x film was observed but the stoichiometry remained almost constant up to a fluence of 3 x 10¹² ions/cm². Structural modifications of the films after irradiation was studied by offline glancing angle X-ray diffraction (GAXRD), which showed the presence of Ge crystallites in the films. The results indicate that the phase separation does not occur because of preferential sputtering of oxygen but may occur as a result of rearrangement of Ge and O atoms within the films. The activation for rearrangement is provided by ion impact