In this work the first-order rate constants of self-decomposition of hydroperoxo and alkylperoxo complexes [Fe(bpr)(2)(OOH)Py](NO3)(2) (2a-Py), [Fe(phen)(2)(OOH)Py](NO3)(2) (2b-Py) and [Fe(bpy)(2)(OOtBu)CH3CN](NO3)(2) (3a-CH3CN) were determined in the presence of various substrates and at various temperatures. It was observed, that the alkylperoxo species are far less stable than corresponding hydroperoxo intermediates, k = 1.2 x 10(-2) s(-1) (3a-CH3CN in CH,CN at - 10 degrees C) and k = 2 x 10(-4) s(-1) (2a-Py in CH3CN at - 10 degrees C). The sixth ligand (Py in 2a-Py and 2b-Py; CH,CN in 3a-CH3CN) can be replaced by other donor molecules B in appropriate solvent systems. Using d(9)-tBuOOH, D-2 NMR signals of tBuOO moieties of complexes 3a-CH3CN, 3a-CH3OH and 3a-H2O were observed. The rate of decomposition of hydroperoxo complexes [Fe(bpy)(2)(OOH)B](NO3)(2) (2a-B), where B are derivatives of Py (3-Br-Py, 3-Me-Py, 4-Me-Py and 4-Me2N-Py) increases with the growth of basisity of B (push effect). Such effect is markedly smaller for alkylperoxo species [Fe(bpy)(2)(OOtBu)B](NO3)(2) (3a-B). The addition of organic substrates (cyclohexane, cyclohexene, methyl phenyl sulfide) in concentrations up to 3 M at -10 degrees C to +20 degrees C does not noticeably change the rate of self-decomposition of 2a-B, [Fe(phen)(2)(OOH)B](NO3)(2) (2b-B) and 3a-B. Thus the intermediates concerned do not directly react with organic substrates. The reactivity patterns of 2a-B, 2b-B and 3a-B were characteristic for free radical oxidation. OH. and HO2. radicals were trapped in solution containing 2a-Py, and tBuOO(.) free radicals were detected in solution in the presence of 3a-B. The determined rates of self-decomposition of complexes 2a-B, 2b-B and 3a-B can be used for evaluation of the upper limit for their reactivity towards organic substrates.