The reactions of carbonate radical anion [systematic name: trioxidocarbonate((.)1-)] with different forms of myoglobin and hemoglobin were studied by pulse radiolysis in N2O-saturated 0.25 M sodium bicarbonate solutions at pH 10.0 and room temperature. The reactions of CO3.- 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, CO3.- generates radical(s) in the globin which then, over a longer time scale, oxidize the iron center to finally produce similar to40% of metHb. The rate constants obtained for the two steps are (2.1 +/- 0.1) x 10(8) and (1.0 +/- 0.2) x 10(2) s(-1), respectively. For the reaction between CO3.- 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 CO3.- 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 CO3.- oxidizes directly both similar to50% of the iron center and amino acid residue(s) of the globin.