The electrochemical behavior of an N-ethyl-N＇-octadecyl viologen (1) assembly on Au and bound to alkanethiol-coated electrodes was studied. The wet and dry self-assembled monolayers (SAMs) of 1 on Au electrodes showed a single redox wave for the first reduction in the presence of KCl, NaNO3, Na2SO4, and NaClO4. On the, other hand,in the presence of NH4PF6, the wet SAM of 1 showed sharp shoulder-reduction peaks at -0.50 and -0.57 V and two oxidation peaks at -0.50 and -0.42 V, and the dry SAM of 1 showed only a single redox wave. On the basis of in situ Fourier transform infrared reflection spectroscopy studies, we assigned the sharp reduction peak of -0.50 V to the reduction of strongly interacted dications of I surrounded by water molecules and the more negative potential reduction peak (-0.57 V) to the reduction of the dications of 1 ion-paired with PF6- ions. Among the two oxidation peaks, the oxidation peak of -0.50 V was assigned to the usual oxidation of the radical cation whereas the oxidation peak of -0.42 V was assigned to the oxidation of the radical cation dimer. Surprisingly, the assembly of 1 showed an irreversible response on long chain n-alkanethiols [CH3(CH2)(n)SH, n = 11, 13, 15, and 17]-coated Au electrodes in the presence of 0.1 M NH4PF6. Meanwhile, transfer of the same electrode to other supporting electrolytes (typically, KCl, Na(2)SQ(4), and NaClO4) gave a clear redox wave. The observed irreversible response of 1 on long chain alkanethiol-coated Au electrodes is explained by the blocking effect of water molecules by the assembly of 1 on alkanethiol-coated Au electrodes in addition to the presence of very weakly hydrated PF6- ions. It is proposed that because of the absence of water molecules in the assembly of 1 on long-chain alkanethiol-coated Au electrodes. the dications of 1 form an "insoluble salt" with PF6- ions at the electrode surface. Whereas the same electrode was transferred to the aqueous solution of hydrophilic anions such as Cl- and SO42-, the dications of 1 were well-solvated due to the ingress of water molecules along with the hydrophilic anions of Cl- or SO42-into the assembly of 1 on alkanethiol-coated electrodes, and thus, it showed a clear redox response. On the other hand, the assembly of 1 on short-chain n-alkanethiol (n < 9)-coated electrodes showed a well-resolved redox wave in the presence of 0.1 M NH4PF6. The ingress/egress of water and anions into/from the assembly of 1 on Au and alkanethiol-coated Au electrodes during the redox reaction were studied by electrochemical quartz crystal microbalance. It is found that the ingress of water molecules into the assembly of 1 on alkanethiol-coated Au electrodes was less than that of 1 on Au electrodes. The ingress of water molecules also strongly depends on the identity of the anion that is present within the monolayer.