The monolayers of thiol-functionalized viologens, i.e., dihexafluorophosphate salts of N-butyl-N＇-(4-mercaptobutyl)-4, 4＇-bipyridinium (C4V2+C4SH) and N-pentyl-N＇-(5-mercaptopentyl)-4,4＇-bipyridinium (C5V2+C5SH), on a polycrystalline gold electrode were characterized in several aqueous electrolyte solutions using cyclic voltammetry and electroreflectance (ER) spectroscopy. The voltammetric response for the viologen dication/radical cation couple (V2+/V.+) in Na2SO4, K2SO4, KPF6, or KF solution was found to be electrolyte anion dependent. However, considerable differences due to the electrolyte anion or the alkyl chain length in the fractional content of dimer forms in the V.+ state and the average orientation of the V.+ moiety, both estimated from the results of ER spectral measurements, were not observed. A large voltammteric peak width was observed in KF solution. Because the voltammetric measurements suggested a strong and weak binding to the viologen moiety respectively for PF6- and F-ions, the dependence of voltammetric response on the concentration of PF6- was investigated quantitatively for a monolayer of C4V2+C4SH in the mixed solutions of KPF6 and KF at a constant ionic strength. It was found that one PF6- ion additionally binds to a viologen moiety upon the oxidation of V.+ to V2+. Th,ratio of the binding constant to V2+ and V.+ was estimated to be approximately 1.2 x 10(4) M-1 at an ionic strength of electrolyte of 0.4 M. The absence of considerable dependence of the PF6- binding on the ionic strength was suggested by voltammetric measurements in the solutions containing solely KPF6. The electron-transfer rate constants for the monolayers were estimated to be no less in the order of magnitude than 1 x 10(4) s(-1).