In this work the rate constants (k(ex)) and the activation parameters (Delta H-up down arrow, Delta S-up down arrow, and Delta V-up down arrow) for the water exchange process on Mn-III centers have experimentally been determined using temperature and pressure dependent O-17 NMR techniques. For the investigations the Mn-III porphyrin complexes [Mn-III(TPPS)S-2](n-) and [Mn-III(TMpyP)S-2](n+) (S = H2O and/or OH-) have been selected due to their high solution stability in a wide pH range, enabling the measurements of water exchange in the case of both diaqua and aqua-hydroxo complexes. We have experimentally demonstrated that the water exchange on Mn-III porphyrins is a fast process (k(ex) approximate to 10(7) s(-1)) of an I-d to I mechanism, strongly influenced by a Jahn-Teller effect and as such almost independent of a porphyrin charge and a trans ligand. This is also supported by our DFT calculations which show only a slight difference in an average Mn-III-OH2 bond found for a positively charged model porphyrin with protonated pyridine groups (2.446 angstrom) and for a simple model without any substituents on the porphyrin ring (2.437 angstrom). The calculated effective charge on the Mn center, which is significantly lower than its formal +3 charge (ca. +1.5 for diaqua; +1.4 for aquahydroxo), also contributes to its substitution lability. The herein presented results are discussed in connection to a possible fast exchanging substrate binding site in-photosystem II and corresponding inorganic model complexes, as well as in the context of a possible inner-sphere catalytic pathway for superoxide dismutation on Mn centers.