Two ethynyl-derivatized isomers of bis(fulvalene)diiron (BFD, 1,1'biferrocenylene) were prepared and covalently attached to glassy carbon electrodes through their ethynyl group by three different electrode modification methods. Cyclic voltammetry and square wave (SW) voltammetry were used to characterize surface coverages of 1.45.5 x 10(10) mol cm(2) the higher of these corresponding to roughly a monolayer based on computation of an idealized closepacking structure for ethynylbis(fulvalene)diiron (EBFD) on a solid surface. In a dichloromethane solution containing a smaller electrolyte anion such as [PF6] or [ClO4] the EBFDmodified electrodes exhibited two quasiNernstian oneelectron oxidations. In contrast the current for the second oxidation process [EBFD](+/2+) was diminished in electrolytes containing one of the large fluoroaryl borate anions [B(C6F5)(4)] or [B(C6H3(CF3)(2))(4)]. The effect was enhanced for electrodes having higher surface coverages being probed at shorter voltammetric time scales. SW voltammetry showed that the diminished currents for [EBFD](+/2+) in largeanion electrolytes are not caused by slow electron transfer. Rather they are attributed to mixed diffusivity of the counteranions at the electrode/solution interface as [EBFD](+) and the anion form the optimum (lowestenergy) configuration of a 1:1 ion pair. The interior transport of the anion required to reach this configuration may be sterically encumbered accounting for the diminished charge transfer observed with electrolytes containing large anions.