This series of papers addresses the role of sorption in heterogeneous catalysis aimed at the removal of organic contaminants (OCs) from water. This first part is focused on the oxidative treatment of OCs by H2O2 catalyzed by activated carbon (AC). The relative reaction rates of compounds with different hydrophobicities and therefore different sorption tendencies on AC (methyl tert-butyl ether, trichloroethene, 2,4,5-trichlorophenol) in the AC/H2O2 system differed drastically from those observed in a classical homogeneous Fenton system. Quantitative considerations that take into account the ratio of the reaction rate constants of MTBE and TCE in the AC/H2O2 system and the homogeneous Fenton system as well as the ratio of their freely dissolved fractions lead to the conclusion that the predominant pathway for the degradation reaction in the AC/H2O2 system is the attack of OH. radicals on the OC fraction that is freely dissolved in the pore volume of the AC. In contrast, the sorbed fraction is nearly unreactive, i.e. protected against radical attack. Quenching experiments with methanol, a strong competitor for reactions with OH. radicals in the solution phase, further confirmed this hypothesis. Consequently, sorption on AC has an adverse effect on the oxidation of OCs via OH. radicals, even though the radicals are formed directly on the AC surface, i.e. in close proximity to the sorbed OCs. © 2004 Elsevier B.V. All rights reserved.