Non-heme iron(III) complexes of 14-membered tetraaza macrocycles have previously been found to catalytically decomp. hydrogen peroxide to water and mol. oxygen, like the native enzyme catalase. Here the mechanism of this reaction is theor. investigated by DFT calcns. at the (U)B3LYP/6-31G* level, with focus on the reactivity of the possible spin states of the FeIII complexes. The computations suggest that H2O2 decompn. follows a homolytic route with intermediate formation of an iron(IV) oxo radical cation species (L.+ FeIV = O) that resembles Compd. I of natural iron porphyrin systems. Along the whole catalytic cycle, no significant energetic differences were found for the reaction proceeding on the doublet (S = 1/2) or on the quartet (S = 3/2) hypersurface, with the single exception of the rate-detg. O - O bond cleavage of the first assocd. hydrogen peroxide mol., for which reaction via the doublet state is preferred. The sextet (S = 5/2) state of the FeIII complexes appears to be unreactive in catalase-like reactions.