We study the importance of the linear optical properties of thin molecular films in their second-order nonlinear optical characterization. Two-beam second-harmonic generation is used to determine the susceptibility components of Langmuir-Blodgett films of terthiophene-vinylbenzoate and yields consistent results only when the linear properties of the films are included in the model. While this is expected for thick multilayer assemblies (similar to 140 nm), we show that the same holds also for a monomolecular layer (similar to 2.8 nm). In particular, we find that the linear response of the monolayer is well described by the refractive index of the thick films. (c) 2005 Elsevier B.V. All rights reserved.