In this paper we report on the vibrational Fourier transform infrared (FT-IR) and Fourier transform Raman (FT-Raman) spectra of a few donor-acceptor-substituted bi- ter-, and quaterthiophenes, as solids. The relevant vibrational information has been interpreted with the help of spectroscopic data previously collected on a model series of alpha,alpha＇-dimethyl end-capped oligothiophenes, in their neutral, radical cationic, and dicationic forms. We have also performed density functional theory (DFT) quantum-mechanical calculations to analyze the effects of the intramolecular charge transfer on the geometries and vibrational spectra of these systems. (C) 2000 American Institute of Physics. [S0021-9606(00)00110-0].