The coplanarity of 2,2’:5’,2"-terthiophene and its six 3’-substituted derivatives, namely 3-phenyl- (13),3’-(4-methoxyphenyl)-(14),3’-(4-cyanophenyl)-(15),3’-(2-thienyl)-(16),3’-(5-methyl-2-thienyl)-(17), and 3’-(4-pyridyl)-2,2’:5’,2"-terthiophene (18), has been studied by UV spectroscopy and compounds 13 and 17 have also been studied by X-ray diffractometry. For comparison, also an isomer of compound 13, 2’-phenyl-2,3’:5’,2"-terthiophene (20), was synthesized and its structure determined by XRD. Compounds 13-15, 17, 18, and 20 have not been previously reported. The UV spectra reveal that the aromatic and heteroaromatic substituents do not appreciably conjugate with the terthiophene backbone. Single-crystal X-ray diffraction analysis of compounds 13 and 17 shows that in the solid phase these compounds are entirely nonplanar, in contrast with parent terthiophene which is known to be strictly planar. The complete analysis of diffractograms is difficult because of the number of rotational conformers present in the solid phase. The conformational equilibrium of compound 13 is frozen at 183 K, as shown by variable temperature H-1-NMR. Oxidation of terthiophene and compounds 13-16 and 18 with Cu(II) in acetonitrile gives solutions which are shown to contain dimers on the basis of visible-near-infrared (vis-NIR) spectra reported previously. Comparison with the spectra of parent terthiophene shows that in the case of cationic dimers the conjugation of the pendent aromatic side group with the main chain is negligible, but the oligothiophene chain has maintained its coplanarity.