The electrical conduction of self-assembled monolayers (SAMs) made from viologen derivatives was measured using ultrahigh vacuum scanning tunneling microscopy (UHV-STM) with a focus on the molecular structural effect on the electrical conduction. For viologen derivative SAMs, resistances through the monolayers increased exponentially with increases in molecular length when the decay constants of transconductance beta were ca. 0.35 to 0.48 nm(-1). The estimated monolayer resistances of the viologen derivatives such as N-methyl-N'-(10-mercaptodecyl)-4,4'-bipyridinium (VC10SH), N-methyl-N'-di(8-mercaptooctyl)-4,4'-bipyridinium (HSC8VC8SH), and N-methyl-N'-di(10-mercaptodecyl)-4,4'-bipyridinium (HSC10VC10SH) SAMs were 1.3 x 10(8) Omega, 3.6x 10(9) Omega, and 3.8x 10(9) Omega, respectively. (c) 2006 Elsevier B.V. All rights reserved.