The dispersion of the second-order optical nonlinearity in chromophoric self-assembled stilbazolium films has been characterized in detail as a function of fundamental wavelength from 800 to 1600 nm using an optical parametric amplifier-based measurement system. The second-harmonic generation (SHG) spectrum exhibits a distinctive two-photon resonance at ω=1.3 eV (960 nm). The maximum in the second-order susceptibility coincides with a low-energy chromophore-centered charge-transfer one-photon excitation at 480 nm (2.6 eV). The experimental SHG dispersion values compare favorably with theoretical results computed using a semiempirical sum-over-states formalism. These indicate that the χ(2) response is dominated by an excitation along the long axis of the stilbazolium chromophore from the highest occupied to the lowest unoccupied molecular orbital.