Postfunctionalization of poly(3-hexylthiophene) (P3HT) enables the systematic study of electronic and steric effects of various functional groups on the optical and photophysical properties of 3,4-disubstituted poly(thiophene)s. In solution, these 3,4-disubstituted poly(thiophene)s exhibit a lower fluorescence yield (Phi(fl)) than P3HT. In the solid state, bromo, chloro, and formyl groups increase Phi(fl), whereas nitro groups completely quench fluorescence. Phenyl, p-tolyl, 2- and 4-methoxyphenyl, 2-thienyl, biphenyl, and 1-naphthyl substituents increase the solid-state fluorescence of P3HT by a factor of 2-3 as a result of steric interactions that force the aromatic substituents perpendicular to the main chain. An extensive enhancement in the solid-state Phi(fl), 12 times greater than that of P3HT, is observed for polymers containing ortho-alkylphenyl or 2-(3-alkyl)thienyl groups, because of additional steric interactions between methyl or alpha-methylene hydrogens on the phenyl or thienyl substituents arid main-chain thienyl groups. These interactions further increase the planarity of the polymer backbone and lead to an enlargement of the interplanar distance. Partial substitution of P3HT with o-tolyl groups affords polymer films with a solid-state Phi(fl) of up to 22%.