Bio-inspired non-heme iron catalysts consisting of iron(II) centers coordinated to tetradentate N4 ligands catalyze conversion of olefin substrates into epoxide and/or cisdiol products by reaction with H2O2 as a final oxidant. The catalytic results, i.e., the activity and diol-to-epoxide ratio, could be modulated by the effect of different additives. An additive such as acetic acid could increase the activity and selectivity to epoxidation. To date, no satisfactory mechanistic insight has been reported to explain its effect. The effect of the presence of acetic acid on the nature of the active oxidant species derived from the reaction of [(TPA)Fe(CH3CN)2]2/+ (TPA = tris(2-pyridylmethyl)amine) with H2O2. Results obtained using a variety of spectroscopic techniques (UV-visible, EPR, Mossbauer and ESI-MS) expanded the initial mechanistic hypothesis, where the HO FeV=O intermediate appeared as the main oxidizing species, to include additional pathways in the presence of acetic acid, where other key intermediates such as FeIII-(OOH) or FeIV=O are implicated. This is an abstract of a paper presented at the 231st ACS National Meeting (Atlanta, GA 3/26-30/2006).