A nanocrystalline TiO2 film electrode on conductive glass was modified with the viologen compound, bis(2-phosphonylethyl)4,4'-bipyridinium dichloride, to form an electrochromic layer. Electrochemical reduction of the modified electrode in 0.1 M LiClO4 gamma-butyrolactone solution is accompanied by a change of colour from transparent to blue. The process was studied by cyclic voltammetry, optical transmittance, electrochemical impedance and modulated transmittance at 370 and 630 nm. The study shows that the dynamic optical responses at the two wavelengths can be used to follow the coupled relaxation of the viologen and electron concentrations in the nanocrystalline layer. (C) 2003 Elsevier Science B.V. All rights reserved.