The heterogeneous rate constant k0 of the ferrocene/ferrocenium (Fc/Fc+) redox couple and the diffusivities (DFc and View the MathML sourceDFc+) of the two species were measured in a toluene/trihexyl(tetradecyl) phosphonium tris(pentafluoroethyl) trifluorophosphate solution. This ionic liquid allowed to increase sufficiently the solution conductivity so that those measurements could be performed using a rotating disk electrode. For a 0.1 mol L−1 ionic liquid concentration, the results obtained from regression on electrochemical impedance data provided: k0 = (4.82 ± 0.38) 10−5 m s−1, DFc = (1.43 ± 0.33) × 10−9 m2 s−1; View the MathML sourceDFc+ = (0.98 ± 0.23) × 10−9 m2 s−1. The capacitance values close to the rest potential were in the range of 1 μF cm−2 in agreement with a nonspecific adsorption of the solvent and with the large size of phosphonium cations.