A study of the time dependence of the radiolytic yields of the hydrated electron (e,,e(aq)(.-)) produced with nanosecond pulses of high linear energy transfer (LET) particles is presented. Pulses of 95 MeV/amu O-16(8+) and Ar-36(18+) are used. Their respective LET values are 50 +/- 1 and 271 +/- 5 eV/nm. The g-values of e(aq)(.-) at 10 ns and even 1 ns, 5.9 x 10(-9) mol J(-1) with Ar18+, are compared to those obtained with C6+. Because of the same velocity of these ions (beta = 0.4), the g-values are interpreted in terms of picosecond-time scale reactions of high concentrations of radicals (0.16 M) in the track cores. (C) 2003 Elsevier B.V. All rights reserved.