The electrochemical properties of the redox mediator Co(III)/Co(II)(dbbip)(2) (dbbip = 2,6-bis(1'-butylbenzimidazol-2'-yl)pyridine) in a mixed acetonitrile/ethylene carbonate solvent have been studied by a range of techniques in order to determine the rate constants for electron transfer and the diffusion coefficients of the Co(II) and Co(III) species. Platinum, gold, fluorine-doped tin oxide (FTO) and compact TiO2 layers were used as electrode materials. The results have been used to predict the limitations imposed on the performance of dye-sensitized nanocrystalline cells by the electron transfer kinetics and mass transport properties of the redox mediator. The Co(III)/Co(II) redox mediator is compared with the conventional triiodide/iodide redox system used in high performance dye-sensitized solar cells. (C) 2004 Elsevier B.V. All rights reserved.