The work deals with Fe-containing zeolites for the catalytic oxidation of water-dissolved organic substances by H2O2. A detailed study of the system FeZSM-5/H2O2 meant to reveal the nature of the catalysis is presented. For this purpose, the following characteristics of the system were determined: the iron concentration in the filtrate of the catalyst suspension in several reaction runs; Fe content changes in the repeatedly used heterogeneous catalyst; the catalytic activities of the filtrate and of the heterogeneous catalyst in several runs. It has been shown that in the system FeZSM-5/H2O2, the heterogeneous catalysis makes the main contribution to the catalyst activity in the H2O2, decomposition and oxidation of organic substrates. Additionally, in the presence of the iron ions complexing agents, FeZSM-5 showed a significantly different behavior in comparison to the homogeneously dissolved iron cations. For example, in the presence of the P2O74-anions, the rate of the H2O2 decomposition increased by several times in the FeZSM-5 suspension, while it is totally reduced in a homogeneous iron solution. (C) 2004 Elsevier B.V. All rights reserved.