Membrane introduction mass spectrometry (MIMS) was used to investigate kinetic and mechanistic aspects of the reaction of benzene derivatives with Fenton's reagent (Fe2+/H2O2) in water. Under the conditions employed, the reaction rate showed a first-order dependence on the aromatic compound concentration. The order of reactivity observed was C6H5Cl > C6H5Br > C6H6 > C6H5CH3 > C6H5OCH3 > C6H5NO2) C6H5OH, and, with the exception of C6H5NO2, a linear Hammett relationship (log k(X)/k(H) versus sigma(p)) was observed. This fact suggests that electronic factors significantly influence reactivity with the Fenton's reagent. Experiments with C6H6 and C6D6 showed the presence of an isotopic effect of k(H)/k(D) = 1.7, suggesting that cleavage of the benzene C-H bond occurs in the reaction rate controlling step. Mechanistic studies with chlorobenzene showed that mineralization to CO2 and chloride proceeds via hydroxylation steps producing phenolic, hydroquinonic, and quinonic intermediates.