The electron transfer reactions between molecules of flavodoxin from Megasphaera elsdenii in different redox states have been investigated by proton nuclear magnetic resonance techniques at 500 MHz. The electron transfer between molecules in the oxidized and semiquinone state is shown to be at least 350-times slower than that between molecules in the semiquinone and hydroquinone state. The latter reaction was studied at different ionic strengths and temperatures. The rate of electron transfer increases with increasing ionic strength, as expected for a reaction between molecules of identical charges. The electron transfer reaction is only slightly dependent on temperature suggesting an outer sphere reaction mechanism. The results indicate that the activation energy for the electron transfer reaction between the semiquinone and hydroquinone state is negligible in contrast to that between the oxidized and semiquinone state. It is suggested that this feature renders M. elsdenii flavodoxin to an exclusive one-electron donor/acceptor in the cell, thereby shuttling between the semiquinone and the hydroquinone state. Mechanistic implications of the findings are briefly discussed.