in this work, an indirect method for estimating the total amount and concentration of oxidative radicals in aqueous and slurry-phase Fenton's systems was developed. This method, based on the use of benzoic acid as probe compound, was applied for evaluating the effect of the operating conditions on the radicals amount produced, their production efficiency (i.e. moles of radicals generated per mole H2O2) and their concentration. A Rotatable Central Composite design (RCC) was used to select the operating conditions in order to get a statistically meaningful data set. Hydrogen peroxide and ferrous ion concentrations ranged between 0.2-1 mM and 0.2-0.5 mM, respectively; humic acid concentration between 0 and 15 mg/L, whereas the soil/water weight ratio in slurry-phase systems between 1:10 and 9:10. The probe compound concentration was 9 or 0.1 mM in experiments aimed to evaluate the total amount or concentration of oxidative radicals, respectively. The obtained results indicated that the amount of radicals generated in both aqueous and soil slurry Fenton's system increased with higher H2O2 concentration and, more specifically, that their production efficiency increased with increasing Fe(II):H2O2 molar ratio. Addition of dissolved organic compounds as humic acid did not notably affect the oxidative radicals amount and concentration. On the contrary, a one order of magnitude reduction in both radicals amount generated and concentration was observed when soil was added to the reaction environment. (C) 2008 Elsevier B.V. All rights reserved.