The beta chains of hemoglobin contain an invarient cysteine at position 93 This is the only reactive sulftiydryl in hemoglobin The involvement of this residue could, therefore, be established by comparing results for normal hemoglobin with those obtained in the presence of N-ethylmaleimide, which covalently blocks the sulfhydryl group. A role in the oxidation of hemoglobin and heme degradation has now been demonstrated. Using low temperature electron paramagnetic resonance we have shown that during autoxidation Superoxide is liberated in the heme pocket. Within the beta chain an electron transfer reaction with the cyseine residue results in the formation of a thiyl radical and the reduction of the Superoxide to peroxide. The formation of peroxide withing the heme pocket due to the reactions of Superoxide place a peroxide in close proximity to the heme. The reaction of peroxides with hemoglobin produce the Fe(IV) ferryl state of hemoglobin which decomposes to produce fluorescent heme degradation products. A detrimental role for peroxide produced during autoxidation is implied by the detection of the same fluorescent bands with emission at 465 nm and 540nm during autoxidation and during the reaction of H202 with Fe(II) hemoglobin.