Electric field induced changes in the optical properties of alkyl substituted polytiiophenes and polythienylevinylenes are studied in the spectral region of the π-π* electronic transition. The spectral shape and the magnitude of the electroabsorption (EA) signal are compared for mono- and di-alkyl substituted polythienylenevinylenes as well as for a well ordered poly(3-alkylthiophene). For all these polymers the induced absorption scales quadratically with the applied electric field and its lineshape is field invariant. For the well ordered poly(3-alkylthiophene) the EA spectrum resembles the first energy derivative of the unperturbed absorption, though there are several additional features. The EA spectra of the alkyl substituted polythienylenevinylenes do not however have a characteristic (e.g. first, second or third derivative) relationship to the unperturbed absorption. © 1993.