The structural and electronic effects of various substituents on thiophene-based oligomers have been studied using a density-functional-based tight-binding scheme. Optical absorption spectra have been calculated applying the semiempirical Hartree-Fock-based ZINDO/S method. Electron withdrawing or donating substituents have been placed on the two terminal thiophene units in alpha, beta, and gamma positions with respect to the sulfur atom of the thiophene. The presence of substituents has in general only minor effects on the geometry of the parent molecules, but strong influence on the optical properties. Substitution in alpha and gamma positions always leads to a redshift. The effect of beta substitution is much weaker and electron withdrawing groups even cause a modest blueshift. The calculated shifts can be explained in terms of modifications of energy and shape of the frontier orbitals.