The kinetics of the electron-transfer reaction between Fe(CN)6-3 and Fe(CN)6-4 ions in aqueous solutions was studied by measuring the N14 n.m.r. line width. The reaction obeys a rate equation: rate = k[Fe(CN)6-3] [Fe(CN)6-4] with k = 9.2 ± 1.3 × 104 sec.-1 M-1 at 32° and an Arrhenius activation energy of 4.2 kcal./mole in the temperature range 25-40°. The effect of replacing the potassium by other alkali and alkaline earth cations is reported. The rate increases from H+ to Cs+ and from Mg+2 to Sr+2. Possible mechanisms, in view of the catalytic effect of the cations, are discussed. The effect of the presence of various cations on the N14 line width is reported and discussed.