The degree of intramolecular charge transfer (ICT) in 9-(1,3-dithiol-2-ylidene)fluorenes bearing fused thiophene and 2,5-dihydrothiophene units (2 and 16, respectively), has been investigated by UV-VIS spectroscopy. Both series of compounds show a good correlation between the ICT energies and Σσ_p^– values of the corresponding molecules. The sensitivity parameters ρ_ICT^– are 0.18 eV for 2 and 0.16 eV for 16. Cyclic voltammetry of both series, 2 and 16, reveals a single irreversible oxidation process in all compounds. However, electropolymerisation under oxidative conditions does not take place, since the radical cation intermediate is most probably derived from the electroactivity of the 1,3-dithiole–fluorene fragment. In parallel to ICT, the values of E_1red^½ and E_2red^½ in relevant compounds also show a linear relationship with σ_p^– constants in the fluorene ring. The sensitivity parameters for series 2 are 0.20 V (ρ_1red^–) and 0.17 V (ρ_2red^–), whilst those for 16 are 0.26 V (ρ_1red^–) and 0.18 V (ρ_2red^–).