An HPLC-ECD method is described for the indirect determination of the hydroxyl (OH) radical. Fenton's reaction is used to produce (OH)-O-., which simultaneously attacks phenols (phenol or pyrocatechol) to form the adducts, pyrocatechol or pyrogallic acid. Thus, [(OH)-O-.] quantification is based on the separation and detection of pyrogallic acid and/or pyrocatechol by an isocratic HPLC-ECD method. The quantification of (OH)-O-. is also performed alternatively by a chronoamperometric detection in an electrochemical cell, where simultaneously formed Fe-III (Fenton's reaction) combines [Fe-II(CN)(6)](4-) to produce the Prussian blue (PB) molecules (Fe-4(III)[Fe-II(CN)(6)](3)). Newly formed PB molecules are then immediately converted to colorless Everitts salt (K4Fe4II[Fe-II(CN)(6)](3)) with the reduction of the high-spin Fe-III to Fe-II at the surface of a glassy carbon electrode at +0.150 V (versus Ag/AgCl]). The calculated concentration of (OH)-O-. during incubation (0.626 ppm) can be detected with negative errors by the HPLC-ECD (0.595 and 0.615 ppm with the errors -5.2 and -1.8%, respectively) and by the chronoamperometric method (0.552 and 0.607 ppm with the errors -11.8 and -3.0%, respectively). For the comparison of the two sets of data, HPLC-ECD method is much more promising. (C) 2004 Elsevier B.V. All rights reserved.