Langmuir-Blodgett (LB) monolayers of (E)-4-[2-(bis(octadecylsulfanyl)tetrathiafulvalenyl)ethenyl]-1-methylpyridinium iodide exhibit an area of 0.53 +/- 0.04 nm(2) molecule(-1) in contact with the substrate and a thickness of 2.1 +/- 0.04 nm, the dimensions being derived from the frequency change when deposited onto a 10-MHz quartz crystal and the surface plasmon resonance data, respectively. The molecules are non-centrosymmetrically aligned, and thus the films show second-harmonic generation (SHG) and have an effective susceptibility of 70 pin V-1 at 1.064 mum.