Voltammetric and spectroscopic data for novel copolymers of fluoren-9-one with benzene, thiophene, and furan have been used to explore the relationship between the formal potentials and optical band gaps of conjugated polymers. It is shown that the optical band gap measured from the onset of absorption corresponds to the difference in the formal potentials for n- and p-doping. Voltammograms have been analyzed to establish how formal potentials should be defined for these and other conjugated polymers. (c) 2006 Elsevier Ltd. All rights reserved.