Low band gap polythiophene derivatives synthesized using the method of Stille coupling have shown good third order nonlinear optical properties. In order to gain more insight into the structure/ property relationship in these materials, we measured the optical properties of a number of oligothiophene derivatives. In addition to linear absorption spectra, we investigated the third order nonlinear optical properties using Z-scan at a number of wavelengths. As in other conjugated oligomers, the properties depend strongly on chain length. We also observe significant dependence on the substituent type, number, and position, with strong enhancement of the oscillator strength for asymmetric substitution. The resonance nonlinear absorption shows strong saturation behavior, in general changing to induced absorption off-resonance. The nonlinear refractive indices were negative near resonance, suggesting the possibility of thermal effects. While off-resonant nonlinear refractive indices were small, in the five substances for which they could be measured, the values were found to be negative, suggesting that resonant enhancement is still playing a role.