The existence of structural water in the interior of both oxidized and reduced horse-heart cytochrome c in solution is demonstrated using nuclear magnetic resonance spectroscopy. Six water molecules have been located in ferrocytochrome c and five in ferricytochrome c, with residence times greater than a few hundred picoseconds. Two water molecules are located in the haem crevice, one of which is found to undergo a large change in position with a change of oxidation state. Both of these observations indicate that buried structural waters in the haem crevice have, by microscopic dielectric effects, significant roles in the setting of the solvent reorganization energy associated with electron transfer.