Diffusion coefficients of nickel and silicon in Fe-20Cr-20Ni under 300-keV Ni⁺ irradiation at temperatures between 293 and about 930 K with an ion flux of 6.3 × 10¹² cm⁻² · s⁻¹ were measured with the SIMS technique. Results indicate that below about 700 K transport is dominated by athermal recoil mixing. At higher temperatures diffusion coefficients increase with increasing temperature. This temperature dependence is discussed with respect to point defect reactions and diffusion. The activation enthalpy of 0.67 eV for nickel diffusion in Fe-20Cr-20Ni is determined by half of the vacancy migration enthalpy. Diffusion coefficients of silicon in the alloy are about one order of magnitude larger than the nickel diffusion coefficients.