[en] We have investigated the production of free radicals induced by swift ions during the radiolysis of oxygenated water and analyzed the underlying mechanisms in detail. To this aim, we simulated, by Monte-Carlo, the irradiation of water by projectiles with LET values ranging from 1 to 300 keV/μm for a partial pressure of oxygen in air from 0 to 750 mmHg, and for times up to 10 μs after ion impact. For low-LET radiation, we observed an increase in production of (HO_2"· + O_2"·"−) with oxygen pressure and a saturation. At 1 μs, the saturation occurred at a pressure of 20–30 mmHg and the maximal yield amounted to 0.3 μmol L"−"1 per Gray. For the same conditions, we observed similar trends for high-LET ions, but we observed a significant reduction in the yield values and an attenuation of the saturation behavior. By underlining similarities between the yield of (HO_2"· + O_2"·"−) and the oxygen effect observed in radiobiology, we discuss the role of (HO_2"· + O_2"·"−) in oxygen effect and suggest a general mechanism for this phenomenon.