The reactions of carbonate radical anion [systematic name: trioxidocarbonate(1-)] with different forms of myoglobin and hemoglobin were studied by pulse radiolysis in N 2 O-saturated 0.25 M sodium bicarbonate solutions at pH 10.0 and room temperature. The reactions of CO 3 _ - with metMb and metHb involve only amino acid residues of the globin and no oxidation of the iron is observed. The second-order rate constants measured are (4.7 ′ 0.3) x 10 7 and (1.9 ′ 0.3) x 10 8 M - 1 s - 1 , for metMb and metHb, respectively. The carbonate radical anion-mediated oxidation of oxyHb proceeds in two steps: First, CO 3 _ - generates radical(s) in the globin which then, over a longer time scale, oxidize the iron center to finally produce ∼40% of metHb. The rate constants obtained for the two steps are (2.1 ′ 0.1) x 10 8 and (1.0 ′ 0.2) × 10 2 s - 1 , respectively. For the reaction between CO 3 _ - and oxyMb, at all wavelengths studied we obtained kinetic traces that could be fitted to a single-exponential expression. Two distinct two step mechanisms were proposed to explain the kinetic data. The reaction of CO 3 _ - with oxyMb proceeds either according to a mechanism identical to that observed for the reaction with oxyHb but with a significantly faster rate of electron transfer from the globin radical(s) to the iron (>6 x 10 4 s - 1 ) or according to a concurring mechanism in which CO 3 _ - oxidizes directly both ∼50% of the iron center and amino acid residue(s) of the globin.