Iron can be a detrimental catalyst in biological free radical oxidations. Because of the high physiological ratio of [O 2 ]/[H 2 O 2 ] (≥10 3 ), we hypothesize that the Fenton reaction with pre-existing H 2 O 2 is only a minor initiator of free radical oxidations and that the major initiators of biological free radical oxidations are the oxidizing species formed by the reaction of Fe 2+ with dioxygen. We have employed electron paramagnetic resonance spin trapping to examine this hypothesis. Free radical oxidation of: 1) chemical (ethanol, dimethyl sulfoxide); 2) biochemical (glucose, glyceraldehyde); and 3) cellular (L1210 murine leukemia cells) targets were examined when subjected to an aerobic Fenton (Fe 2+ + H 2 O 2 + O 2 ) or an aerobic (Fe 2+ + O 2 ) system. As anticipated, the Fenton reaction initiates radical formation in all the above targets. Without pre-existing H 2 O 2 , however, Fe 2+ and O 2 also induce substantial target radical formation. Under various experimental ratios of [O 2 ]/[H 2 O 2 ] (1-100 with [O 2 ] ≈ 250 μM), we compared the radical yield from the Fenton reaction vs. the radical yield from Fe 2+ + O 2 reactions. When [O 2 ]/[H 2 O 2 ] < 10, the Fenton reaction dominates target molecule radical formation; however, production of target-molecule radicals via the Fenton reaction is minor when [O 2 ]/[H 2 O 2 ] ≥ 100. Interestingly, when L1210 cells are the oxidation targets, Fe 2+ + O 2 is observed to be responsible for formation of nearly all of the cell-derived radicals detected, no matter the ratio of [O 2 ]/[H 2 O 2 ]. Our data demonstrate that when [O 2 ]/[H 2 O 2 ] ≥ 100, Fe 2+ + O 2 chemistry is an important route to initiation of detrimental biological free radical oxidations.