Third-harmonic generation measurements have been carried out to estimate third-order nonlinear optical susceptibilities for a series of alkyl-and alkoxy- substituted polythiophenes. χ(3) (−3ω; ω, ω, ω) values were found to be in the range 3.6×10−12 to 1.2×10−11 esu. The phase of χ(3) indicated contributions from two-photon absorption. Substituent effects on χ(3) were found to be minor. Possible enhancements of nonlinear optical properties of this class of materials, including film preparation, the nature of the substituents and polymer conformation are discussed.