The title compound, 2,4-bis(N-octadecylpynol-2-yl)squaraine (1a), is centric and shows no discernible second-harmonic generation (SHG) when deposited as a Langmuir-Blodgett (LB) film. This conflicts with the disclosure of second-order activity by Lynch et al. (D.E. Lynch, U. Geissler, I.R. Peterson et al., J. Chem. Sec. Perkin II (1997) 827) and invalidates the IR proposed mechanism, the SHG being impurity-induced and influenced by interfacial effects. Their data indicate a mixture of isomeric forms, the centric 2,4-squarate (1a: nu(CO) = 1625 cm(-1)) and an acentric 1,2-squarate (1b: nu(CO) = 1750 cm(-1)) in a ratio of 3:1, whereas our SHG-inactive sample has a single CO band at 1626 cm(-1). LB films of the related dye, 2,4-bis(N-octadecyl-3,5-dimethylpyrrol-2-yl)squaraine (2), exhibit very weak SHG when deposited on glass slides but the signal is suppressed when there is an organic buffer layer between the monolayer and substrate. The non-linear optical behaviour is not an inherent property of the film and, instead, is attributed to the polarization of the chromophore by the underlying substrate. The thickness and dielectric permittivities of the LB monolayer of dye 2, derived from an analysis of the surface plasmon resonance (SPR) at 632.8 nm, are l = 27 Angstrom, epsilon(r) = 3.4 and epsilon(i) = 0.39, respectively.