The diffusion of Co(DTB) 3 n+ (where DTB ) 4,4′-di-tert-butyl-2,2′-bipyridine) in dye-sensitized solar cells (DSSCs) was investigated. Current vs time experiments reveal that DSSCs with cobalt complex-based mediators display a qualitatively different response to the onset of illumination than do DSSCs with I -/I 3 -, exhibiting a sharp decline in the initial current over ∼1 s or less. The results of these and other experiments conducted on DSSC sandwich cells are consistent with insufficient mass transport of Co(DTB) 3 3+ to the cathode, resulting in solution polarization and limited photocurrents. Rotating disk electrode voltammetry on Co(DTB) 3 n+ and I 3in acetonitrile was performed to compare the rates of diffusion within bulk solution and typical mesoporous titania films. After accounting for viscosity differences, it was found that the effective diffusion coefficient of Co(DTB) 3 n+ through typical titania films in DSSCs is 0.5-3 × 10 -7 cm 2 s -1 , about 1 order of magnitude slower than I 3 -. This difference may be attributed to the greater size and slower bulk diffusion of Co(DTB) 3 n+ , greater viscosity of cobalt complex solutions, and a possible electrostatic surface interaction of Co(DTB) 3 n+ within the TiO 2 film.