The treatment of thiophene and 3-alkylthiophenes with thallium(III) trifluoroacetate (TTFA) in trifluoroacetic acid (TFA) gives insoluble and dark powdery solids with oxygen content and electrical conductivities ranging from 10-4 to 10-6 Ω-1 cm-1. Polar and short fractions are negligible. All of them show semiconductivity (10-3−10-6 Ω-1 cm-1) when doped in iodine atmosphere. Electron paramagnetic resonance (EPR) spectra of either as-synthesized or I2-treated solids display characteristic single and broad lines (ΔHpp, 1.84−7.4 G) with Lorentzian shapes and g-values in the range 2.0028−2.0038. Infrared spectra show characteristic C−H out-of-plane deformations (780 cm-1 for polythiophene and 820−825 cm-1 for poly(3-alkylthiophenes)) in addition to a strong peak at 1650−1690 cm-1 which has not been conclusively assigned. EPR spectra of some disubstituted and tetrasubstituted 2,2‘-bithiophene radical cations have been observed and their g-values and coupling constants assigned when the corresponding parent compounds are photolyzed with ultraviolet light in TFA. Photolysis of 3-alkylthiophenes in TFA in the EPR instrument gave the radical cations of 4,4‘-dialkyl-2,2‘-bithiophenes. In no case, were EPR signals of the isomeric 3,3‘-dialkyl- or 3,4‘-dialkyl-2,2‘-bithiophene radical cations observed, indicating that dimerization of 3-alkylthiophenes occurs through the less sterically hindered 5-position. The presence of two doublet species corresponding to both conformers, syn and anti, in the radical cations is associated with a large barrier to rotation about the C(2)−C(2‘) bond.