The electronic structure of poly(thiophene-3-methyl acetate) has been investigated using UV–vis absorption spectroscopy and quantum mechanical calculations. Experimental measures in chloroform solution indicatethat the π-conjugation length increases with the polymer concentration, which is reflected by the red shift of the absorbance peak of the π-π* transition. On the other hand, the energy required for the π-π* transition has been found to decrease with the volatility of the solvent forconcentrated polymer solutions, even though the influence of the solvent is very small for dilute solutions. Quantum mechanical calculations indicate that the interactionsbetween the π-conjugated backbone and the methyl acetate side groups are very weak. On the other hand, the lowest energy transition predicted for an infinite polymer chainthat adopts the anti-gauche and all-anti conformations is 2.8 and 1.9 eV, respectively. Finally, measurements on spincasted nanofilms reflect that the π-π* transition energy increases with the thickness, which has been attributed tothe distortion of the molecular conformation. In spite ofthis, the energy gap obtained for the thinnest film (1.52 eV) is significantly smaller than that determined for dilute andconcentrated chloroform solutions (2.56 and 2.09 eV, respectively)