Linear and third-order nonlinear optical (NLO) phenomena have been studied in three thiophene-containing polyimine classes. A complete characterization has been performed determining the real part of the refractive index n, its imaginary part k, and the third-order NLO value ${\chi }^{\left(3\right)}$ . The NLO values derived for the polymers are related to both the electronic property and backbone conformational arrangement, affecting the overall conjugation. The highest third-order NLO coefficient has been measured on a very homogeneous spin-coated film of dialkyl substituted polyazine, showing a value of $5.2\times {10}^{-11}\mathrm{esu}$ at $1710\mathrm{nm}$ , where the material is essentially transparent.