Monolayers of viologens (N-alkyl-N′-(n-thioalkyl)-4,4′-bipyridinium bromide, n ) 6, 8, 10) have been selfassembled from ethanolic solution onto gold substrates. Their structure and redox functionality were studied by cyclic voltammetry and in situ ATR-SEIRAS. Voltammetric experiments revealed that the first redox process, V 2+ T V •+ , is reversible, whereas the second redox process, V •+ T V 0 , is quasi reversible. The V 2+ adlayer exhibits a sandwich-like structure. Van der Waals interactions between the alkyl chains lead to two well-ordered hydrophobic layers, with the redox-active bipyridinium unit enclosed in between. The alkyl chains are aligned in an all-trans configuration in a tilted orientation with respect to the surface normal and exhibit a slight conformational disorder. The long axis of the central bipyridinium rings is also tilted. The alignment of V 2+ is stabilized by interplanar π-π stacking and electrostatic coulomb interactions. Both monomers and dimers of V •+ coexist in the V •+ adlayer. The molecular alignment of the V •+ adlayer is stabilized by the strong vibronic coupling within the dimers and the π-π stacking between the monomers. Increasing length of the alkyl chain results in more tilted bipyridinium units, and favors both the dimerization and the alignment of V •+ within the SAMs. The complex reaction, V 2+ f V •+ , is described as electron transport from the electrode to the V 2+ moiety followed by the dimerization of V •+ , the delocalization of the radical electrons, and the migration of coadsorbed ClO 4anions.