Hydrogen peroxide (H2O2) contributes to oxidative stress by reacting with Fe2+ ions (Fenton reaction) to produce hydroxyl radicals. Blood contains iron, thus levels of H2O2 are affected by its reaction with the Fe2+. Citrate and EDTA chelate Fe2+ and studies show that the reactions of H2O2 with iron complexes of EDTA and citrate are affected by modulation of oxidative stress. The effects of citrate and EDTA anticoagulants on H2O2 levels in blood were investigated. It was hypothesized that EDTA- blood is not suitable to measure H2O2 since EDTA inhibits H2O2-Fe2+ Fenton reaction. Thirty blood samples were collected from sheep from the Berrima Veterinary Laboratory into EDTA and citrate tubes. H2O2 was determined by Biotech H2O2 560 Assay. EDTA: iron ratio was estimated to aid explain measurement of H2O2 in EDTA-anticoagulated blood by using sheep reference ranges for haemoglobin and serum iron. Results revealed no measurable H2O2 in the EDTA blood samples but there was a reaction in citrate samples possibly implicating EDTA in H2O2 inhibition. With the assumption that inhibition of oxidation by EDTA is via inhibition of the H2O2-Fe2+ reaction, this could explain why H2O2 was not detectable in EDTA-anticoagulated blood. It is speculated that EDTA-anticoagulant is unsuitable for testing H2O2 in plasma since level of H2O2 is dependent on EDTA: iron ratio. In the effort to translate oxidative stress into clinical practice, further studies are needed on the effects of different anticoagulants on oxidative stress measurements