Thiophene/phenylene co-oligomers that include the thiophenes and phenylenes with various arrangements in the molecule were investigated spectroscopically in solutions and compared with oligothiophenes and oligophenylenes. The electronic spectra are influenced by both the molecular size and the difference in the thiophene/phenylene arrangements. In the dilute regime (similar to 10(-5) M) monomeric emissions prevail. Quantum efficiencies bf the fluorescence emissions of the co-oligomers are estimated in this regime. High quantum efficiencies of 0.79 and 0.74 were recorded for 4,4＇-bis(2-thienyl)biphenyl (T2P) and 1,4-bis (5-phenylthiophene-2-yl)benzene (AC5), respectively. The experimental data in the dilute regime are compared with the results of the quantum chemical calculations at semiempirical levels. Intermolecular interactions increasingly play a role for higher concentrations. The spectroscopic characteristics in this regime are closely inspected using 2,5-diphenylthiophene (P1T), 5,5＇-diphenyl-2,2＇-bithiophene (P2T), and 1,4-bis(2-thienyl)benzene (T1P). By increasing the concentration, these co-oligomers show well-structured emissions that are red-shifted relative to the monomeric emissions, New absorption shoulders appear in the longer wavelengths region due to the intermolecular interaction. These features are assigned to the intermolecular ground-state complexes with fully overlapped pi-pi groups. Besides the above spectral features, TIP exhibits a long tail toward similar to 700 nm in the absorption spectra at higher concentrations and a broad emission band around 520 nm replaces the strong band at 450 nm. We referred these features to the intermolecular charge-transfer from a thiophene to a phenylene ring. The fluorescence emissions of the co-oligomer thin films are also red-shifted relative to the monomeric emissions. The origin of these emissions in the solid state is briefly discussed.