Transient absorption spectroscopy was employed to study charge-transfer dynamics in dye-sensitized nanocrystalline solar cells (DSSC) containing a new one-electron redox mediator, cobalt(II)-bis[2,6-bis(1'-butylbenzimidazol-2'- yl)pyridine]. Photovoltaic cells incorporating this relay have yielded light-to-electricity power conversion efficiencies of up to 5.2%. This rivals the performance of the tri-iodide/iodide couple that is currently almost exclusively used in DSSC. Interception of the dye oxidized state by electron transfer from the Co(Il) complex in diluted electrolyte was found to follow a first-order kinetics with a rate constant of k(m) = 5 x 10(5) s(-1). Above a threshold of 10(-2) M, under which the cationic relay is essentially adsorbed onto the negatively charged particle surface, larger concentrations of the reduced mediator resulted in a linear increase of the apparent rate, yielding a second-order rate constant of k(m)" = 2.9 x 10(6) M-1 s(-1). Dynamics of the recombination reaction between injected conduction band electrons and the oxidized relay species was monitored spectroscopically, and a first-order rate constant of k(r) = 3 x 10(3) s(-1) was determined directly. This kinetic behavior compares approximately with that of the tri-iodide/iodide redox couple. Practical advantages of the Co-III/Co-II complex mediator, however, make it a promising candidate to replace this widely used system in DSSC.