Newly synthesized sexithiophenes, di- and tetramethylated at the P-positions, are shown to be soluble and processable compounds, giving single crystals suitable fur X-ray structure determination. The molecular packing was characterized in terms of crystal cohesion and short C-H ... S and C-H ...pi intermolecular distances. In particular, the dimethylated sexithiophene displayed very compressed molecular packing and a thin film field effect transistor, fabricated with this material, was characterized by high charge mobility [2 x 10(-2) cm(2)/(V s)]. The tetramethylated compound crystallizes at room temperature in two different systems and with different conformations. The conformational polymorphs, which are easily interconverted at room temperature, are characterized by different wavelengths of light emission and excitation decay rates.