4-{2-[N-(10-Thiodecyl)quinolinium-4-yl]vinyl}phenolate self-assembles on gold with a contact area of 0.35 ± 0.03 nm2 molecule−1, monolayer thickness of 1.64 ± 0.07 nm, and dielectric permittivity components of er ≈ 2.8 and ei ≈ 0.6 at 632.8 nm, which are reduced to ca. 2.0 and 0 respectively when exposed to an acidic medium. The films undergo a change from purple (merocyanine form) to yellow (protonated form) and, by monitoring changes in the reflectance, may be used as sensors with a detection limit of <1 ppm for NH3 in a carrier gas. Langmuir–Blodgett (LB) films of the N-octadecyl analogue show similar behaviour but, for sensing applications, are disadvantaged because the phenolate group is adjacent to the substrate. They have a contact area and monolayer thickness of 0.46 ± 0.03 nm2 molecule−1 and 1.75 ± 0.10 nm respectively, the dimensions indicating that the molecules are either less closely packed or more tilted compared with those of the self-assembled film.