alpha,alpha’-dimethyl substituted oligothiophenes with increasing chain length (from the dimer up to the hexamer) were recently synthesized by chemical methods. In this paper we have investigated the vibrational Fourier transform-IR and Fourier transform-Raman spectra of solid alpha,alpha’-dimethyl substituted oligothiophenes in the neutral state. The data are consistent with the existence of a chain-length dependent pi electron delocalization : a large frequency dispersion with conjugation length is observed for some Raman and infrared active vibrational modes, particularly at the high-energy side of the aromatic C=C stretching region. Vibrational assignments are proposed for the main vibrational features in the whole spectral range. This vibrational spectroscopic analysis of the solid samples thus becomes a tool for deriving information on the structure of these neutral materials in solution and in the doped state.