The effect of temperature and pressure on the water-exchange reactions of complexes of the type [Fe-III(L)(H2O)(x)](n-), where L = edta(4-) (ethylenediaminetetraacetate), Hedta(3-) (monoprstonated form of edta), cdta(4-) (trans-1,2-diaminocyclohexanetetraacetate), edds(4-) (s,s-ethylenediaminedisuccinate), 1,3-pdta(4-) (1,3-propylenediaminetetra-acetate), and alpha,beta -eddadp(4-)-(alpha,beta -ethylenediaminediaceatedipropionate), was studied by employing O-17 NMR techniques; The effect of potentially hexadentate ligands, covering a systematic variation of the size, substituents, and overall coordination geometry, on iron(III) complexes was investigated in terms of the lability of the coordinated water and the underlying exchange mechanism. For most of the systems studied, the results are in agreement with a:dissociatively activated water-exchange mechanism for the seven-coordinate complexes. The absolute magnitudes of the volumes of activation are small and fit an I-d mechanism. The results contribute to a better understanding of the :nature, reactivity, and substitution mechanism of the selected complexes in solution.