Unexpected nonadditivity of currents encountered in the electrochemistry of mixtures of ferrocene (Fc) and cobaltocenium cation (Cc(+)) as the PF6_- salt has been investigated by direct current (dc) and Fourier-transformed alternating current (ac) cyclic voltammetry in two aprotic (1-butyl-3-methylimidazolium tetrafluoroborate and 1-butyl-3-methylimidazolium hexafluorophosphate) and three protic (triethylammonium formate, bis(2-hydroxyethyl)ammonium acetate, and triethylammonium acetate) ionic liquids (ILs). The voltammetry of the individual Fc(0/+) and Cc(+/0) couples always exhibits near-Nernstian behavior at glassy carbon and gold electrodes. As expected for an ideal process, the reversible formal potentials and diffusion coefficients at 23 +/- 1 degrees C in each IL determined from measurement on individual Fc and Cc(+) solutions were found to be independent of electrode material, concentration, and technique used for the measurement. However, when Fc and Cc(+) were simultaneously present, the dc and ac peak currents per unit concentration for the Fc(0/+) and Cc(+/0) processes were found to be significantly enhanced in both aprotic and protic ILs. Thus, the apparent diffusion coefficient values calculated for Fc and Cc(+) were respectively found to be about 25 and 35% larger than those determined individually in the aprotic ILs. A similar change in the Fc(0/+) mass transport characteristics was observed upon addition of tetrabutylammonium hexafluorophosphate (Bu4NPF6), and the double layer capacitance also varied in distinctly different ways when Fc and Cc(+) were present individually or in mixtures. Importantly, the nonadditivity of Faradaic current is not associated with a change in viscosity or from electron exchange as found when some solutes are added to ILs. The observation that the H-1 NMR T-1 relaxation times for the proton resonance in Cc(+) also are modified in mixed systems implies that specific interaction with aggregates of the constituent IL ionic species giving rise to subtle structural changes plays an important role in modifying the mass transport, double layer characteristics, and dynamics when solutes of interest in this study are added to ILs. Analogous voltammetric changes were not observed in studies in organic solvent media containing 0.1 M added supporting electrolyte. Implications of the nonadditivity of Faradaic and capacitance terms in ILs are considered.