The ruthenium complex bis(2,2'-bipyridine-4,4'-dicarboxylic acid) (tetrachlorocatecholato)-ruthenium(II) has been used to modify a thin nanocrystalline transparent layer of antimony-doped SnO2 on a conducting glass electrode. The surface-bound complex shows promise as the basis for an electrochromic window operating in the near infrared region. It undergoes a reversible ligand-centered catecholate/semiquinone oxidation, and the oxidised form has a metal to ligand charge-transfer band transition in the near IR. The redox process in the adsorbed layer causes a change of colour from blue-grey (reduced) to pink (oxidised), and the increase in transmission in the visible (630 nm) is accompanied by a decrease of transmission in the near infrared region (940 nm). The electrochromic system has been studied by cyclic voltammetry, electrochemical impedance spectroscopy and frequency-resolved potential-modulated transmittance at 630 and 940 nm. The results show that the speed of the electrochromic switching process appears to be limited by the RC time constant of the system rather than by the rate constant for electron exchange. (C) 2003 Elsevier Science B.V. All rights reserved.