This Letter reports a sensitive method using HPLC-Fluorescence detection of *OH in liquid water under high-energy heavy ion irradiation. The coumarin-3-carboxylic-acid (3CCA) molecule was selected for probing *OH and providing the fluorescent 7-hydroxy-coumarin-3-carboxylic-acid (7OH-3CCA). Since the concentration limit achievable is better than 1 nM, the radiolytic yields were determined with a sensitivity of 2 x 10(9) mol/J for 4.8 GeV-C-12(6+) of and 20 GeV-Ar-40(18+) in the ns time range. They decrease with the linear energy transfer from 2.8 x 10(7) to 1.3 x 10(7) mol/J (C6+ of 11 eV/nm) and 1.5 x 10(7) to 0.9 x 10(7) mol/J (Ar18+ of 90 eV/nm) which is in agreement with the literature data. (C) 2008 Elsevier B.V. All rights reserved.