The electrospray ionisation mass spectrum of the title compound, 2,4-bis[6-(N-octyl-2,3,4-trihydroquinoloyl)]squaraine, shows peaks which correspond to the monomer (m/z = 569, [2M + H](+)) and aggregate (m/z = 1137, [2M + H](+)). It associates in dilute solution and the nonlinear optical behaviour is attributed to the aggregate, which must be non-centrosymmetric, rather than the molecule. The second-harmonic intensity from solid solutions of the dye in poly(vinyl acetate) increases with concentration. Its homomolecular Langmuir-Blodgett (LB) films also exhibit second-harmonic generation (SHG) and have an effective monolayer susceptibility of 50 pm V-Al at 1.064 mum. The thickness and real and imaginary components of the dielectric permittivity, derived from an analysis of the surface plasmon resonance at 632.8 nm, are l = 0.6 nm layer(-1), e(r) = 1.9 and e(i) = 1.0. The thickness suggests that the chromophore and the hexyl groups align parallel to the substrate.