Picosecond third harmonic generation (THG) spectroscopy in the range 0.9−2.4 μm (0.52−1.38 eV) and sum-over-states (SOS) theoretical modeling have been used to investigate the third-order nonlinear optical properties of a series of nine conjugated aromatic polyimines designed to elucidate structure−χ(3) relationships in conjugated polymers. The THG-measured frequency-dispersed third-order susceptibility (χ(3)) of the series of polymers has off-resonant and three-photon resonance-enhanced values in the ranges of 1.1 × 10-12 to 2.0 × 10-11 and 6.4 × 10-12 to 7.2 × 10-11 esu, respectively, demonstrating a large modulation of nonlinear optical response by simple structural variations. A random copolymer was found to have enhanced χ(3) compared to either of its parent homopolymers. Asymmetric donor side-group substitutions were found to result in 7−10-fold enhancement of off-resonant χ(3) compared to either symmetric substitutions or no substitutions. Theoretical three- and four-level SOS models, justified by site-selective fluorescence spectroscopy, were found to describe the χ(3) dispersion data and the observed multiphoton resonances very well. The zero-frequency χ(3) was predicted from the SOS models to be negative for eight of the nine polymers and to be in the range of −0.43 to −8.12 × 10-12 esu, the magnitudes of which are very close to the observed off-resonance values at 2.4 μm (0.52 eV). A negative real part of χ(3)(−ω;ω,−ω,ω) over a wide wavelength range of λ > 1.38 μm was also predicted by the SOS models for the same eight polymers with strong one-photon and two-photon resonances, suggesting that they would exhibit self-defocusing phenomena. Similarly predicted χ(3)(−ω;ω,−ω,ω) spectra of the poly(azomethines) suggest that third-order susceptibility measured by degenerate four-wave mixing would be up to 2 orders of magnitude larger than the values measured by THG spectroscopy.