The adsorption of various monomethylated pi-extended viologen compounds onto self-assembled layers of octadecylmercaptan (ODM) on gold was investigated with secondary ion mass spectroscopy (SIMS) and atomic force microscopy (AFM). SIMS results indicated that the monomethylated viologen 5-(4-pyridyl)-5'-[4-(N-methyl)pyridinio]-2,2'-bithiophene iodide (PT2) adsorbed much more strongly onto the ODM self-assembled layers at 50 mu M adsorption concentration in ethanol, while very similar compounds were much less effective in their adsorption even at much higher adsorption concentrations. The formation of multilayers of PT2 could be observed by AFM. Some structural features of the viologens, (dipole moment and polarizability), were calculated with semi-empirical quantum chemical calculations (MOPAC/ AMI). One compound, PF2, having a slightly larger molecular length, dipole moment and polarizability than PT2, did not assemble noticeably onto the surface due to its larger solubility. The large difference in efficiency of adsorption can be related to a combination of length and absence of hydrophilic groups at the free pyridine side of the monomethylated viologens,