We present a novel methodology that combines a theoretical description and experimental characterization of the structure and first hyperpolarizability of ultrathin films of VO-octaethyl-porphyrins (VOOETP, Scheme 1) adsorbed on a BK7 activated glass. Films were characterized by ellipsometry, and the geometry is inferred from a consistent comparison between the theoretical hyperpolarizability and the experimental measurements. The hyperpolarizability is determined through second harmonic generation (SHG) techniques, and ellipsometry is used independently to determine the thickness of the film. An analysis of the Fourier components of the tensor relating the incident and SHG fields reveals the existence of a linear dependence between the Fourier components that greatly reduces the amount of required experimental information. Using this methodology, we obtain reasonable agreement between the observed and calculated intrinsic hyperpolarizabilities (beta(z)(exp) / beta(z)(theo) = 0.95) that we attribute to the dimer.