The interaction of the cationic hydroxo-bridged binuclear iron(III) complexes of the sevendentate ligands HPTP and HPTB with montmorillonite was studied. Both complexes can be introduced reversibly into the interlayer region of the smectite. The catalase active HPTP-complex preserves its activity in the smectite interlayers. The unstable peroxide adducts of both complexes can be introduced into montmorillonite interlayers from water/H 2 O 2 at 273 K and can be stored there for months at 243 K. Adduct/montmorillonite compounds can also be prepared by direct reaction of H 2 O 2 with the complexes already bound to montmorillonite. From IR spectra of the adduct/montmorillonite compounds it can be concluded that the adduct of the HPTP complex contains nitrate whereas the adduct of the HPTB complex does not. These results can help to decide whether structures of the adducts proposed in the literature are possible or not and may therefore open a new field of application of clay minerals in studies on unstable intermediates.