We have studied the reaction kinetics of ten manganese porphyrins, differing in their meso substituents, with peroxynitrite (ONOO-) and carbonate radical anion (CO-.3) using stopped-flow and pulse radiolysis, respectively. Rate constants for the reactions of Mn(III) porphyrins with ONOO- ranged from 1 × 105 to 3.4 × 107 M-1S-1 and correlated well with previously reported kinetic and thermodynamic data that reflect the resonance and inductive effects of the substituents on the porphyrin ring. Rate constants for the reactions of Mn(III) porphyrins with CO-.3 ranged from 2 × 108 to 1.2 × 109 M-1S-1 at pH ≤ 8.5 and increased with pH as a consequence of the ionization of the complexes. Mn(II) porphyrins reacted with CO-.3 with rate constants ranging from 1 × 109 to 5 × 109 M-1S-1 at pH 10.4. Hence, fast scavenging of ONOO- and CO-.3 by manganese porphyrins could occur in vivo because of the catalytic reduction at the expense of a number of cellular reductants. Additionally, we determined the pKa of the axial water molecules of the Mn(III) complexes at pH 7.5-13.2 by spectrophotometric titration. Results were consistent with two acid-base equilibria for most of the complexes studied. The pKa values also correlated with the resonance and inductive effects of the substituents. The correlations of E1/2 with the rate constants with ONOO- and with the pKa values display a deviation from linearity when N-alkylpyridinium substituents included N-alkyl moieties longer than ethyl, which is interpreted in terms of a decrease in the local dielectric constant.