The activation parameters and the rate constants of the water-exchange reactions of Mn^IIITE-2-PyP⁵⁺ (meso-tetrakis(N-ethylpyridinium-2-yl)porphyrin) as cationic, Mn^IIITnHex-2-PyP⁵⁺ (meso-tetrakis(N-n-hexylpyridinium-2-yl)porphyrin) as sterically shielded cationic, and Mn^IIITSPP³⁻ (meso-tetrakis(4-sulfonatophenyl)porphyrin) as anionic manganese(III) porphyrins were determined from the temperature dependence of ¹⁷O NMR relaxation rates. The rate constants at 298 K were obtained as 4.12 × 10⁶ s⁻¹, 5.73 × 10⁶ s⁻¹, and 2.74 × 10⁷ s⁻¹, respectively. On the basis of the determined entropies of activation, an interchange-dissociative mechanism (I_d) was proposed for the cationic complexes (ΔS^‡ = ∼0 J mol⁻¹ K⁻¹) whereas a limiting dissociative mechanism (D) was proposed for Mn^IIITSPP³⁻ complex (ΔS^‡ = +79 J mol⁻¹ K⁻¹). The obtained water exchange rate of Mn^IIITSPP³⁻ corresponded well to the previously assumed value used by Koenig et al. (S. H. Koenig, R. D. Brown and M. Spiller, Magn. Reson. Med., 1987, 4, 52–260) to simulate the ¹H NMRD curves, therefore the measured value supports the theory developed for explaining the anomalous relaxivity of Mn^IIITSPP³⁻ complex. A magnitude of the obtained water-exchange rate constants further confirms the suggested inner sphere electron transfer mechanism for the reactions of the two positively charged Mn(III) porphyrins with the various biologically important oxygen and nitrogen reactive species. Due to the high biological and clinical relevance of the reactions that occur at the metal site of the studied Mn(III) porphyrins, the determination of water exchange rates advanced our insight into their efficacy and mechanism of action, and in turn should impact their further development for both diagnostic (imaging) and therapeutic purposes.