The adsorption of the oxidized and the reduced forms of lumiflavin (-0.150 V and -0.450 V, respectively) at pH 4.5, and the electrochemical conversion of them were investigated. Two adsorbed surface stages could be distinguished for each oxidation state. Vertical interactions between adsorbed molecules lead to bilayer formation at high surface concentration. These interactions are stronger for the reduced form (H2LF) than the corresponding oxidized (LF) molecule. Surface charge values for a virtual monolayer of Q(M) = 17 mu C cm(-2) for both, LF and H2LF, and the corresponding capacitances of 16 mu F cm(-2) and 11 mu F cm(-2) were obtained, respectively. The electrochemical behaviour of adsorbed species under sub-monolayer coverage is characterized as a quasi-reversible redox process conditioned by molecular lateral interactions. The apparent standard rate constant for the overall two-electron process, k(0) = 295 s(-1), and a transfer coefficient, a = 0.5, were estimated from square wave voltammetry. (c) 2006 Elsevier B.V. All rights reserved.