[en] In this study, the interaction of energetic oxygen ions with the beryllium-tungsten alloy Be₂W is investigated by depth-resolved x-ray photo-electron spectroscopy. A 4 nm tungsten layer is deposited on polycrystalline beryllium. By alloying at 900 K for 60 min, Be₂W is formed. In successive steps, oxygen ions are implanted with 500 and 1000 V acceleration voltage with a fluence of 5×10¹⁴ cm^-2 in each step. Between the single implantation steps the sample is heated for 30 min at 600 K. Depth-resolved photoelectron spectroscopy is performed after each experimental step. The W 4f and Be 1s spectra reveal the formation of beryllium-tungstate BeWO₄ already at room temperature (RT) in significant amounts due to energy deposition of implanted oxygen ions. No indication of oxygen diffusion at RT is observed. In previous experiments in thermal equilibrium, the ternary compound has proven to be stable at temperatures up to 1073 K. Contrary to these results, annealing at 600 K leads to decomposition of the BeWO₄ formed by energetic oxygen ions. After decomposition at 600 K, oxygen stays in the sample bound as BeO.