The isotherms of E-N-alkyl-4-[2-(4-(N-octadecyl-N-ethylaminophenyl))-ethenyl]-pyridinium halide, represented here by CmH2m+1-D-pi -A(+)-CnH2n+1X-, suggest that this two-legged molecule initially adopts a "U-shaped" conformation at the air-water interface but becomes "stretched" as the surface pressure is increased. The area of 0.25 +/- 0.05 nm(2) molecule(-1), prior to collapse, is independent of the second alkyl group and approximates to the molecular cross-section. When deposited, the area in contact with the substrate is 0.25 +/- 0.05 nm(2) molecule(-1) and the laver thickness compares favourably with the molecular length. The second-harmonic intensity is strong when the alkyl chain lengths are sufficiently different, i.e. \(m - n)\ greater than or equal to 4, but is suppressed when m approximate to n. In this limiting case, the second-order properties suggest a centrosymmetric arrangement and, therefore, a random orientation of the molecular dipoles, up and down, within the monolayer film.