The electrode reaction of tris(2,2'- bipyridine) cobalt complex, [Co(bpy)(3)](3+/ 2+), has been investigated in 1-butyl-1-methyl-pyrrolidinium bis(trifluoromethylsulfonyl) amide (BMPTFSA) and 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIBF4). Diffusion of [Co(bpy)(3)](2+) and [Co(bpy)(3)](3+) was affected by the viscosity and the charge densities of the complex and the anion of ionic liquid. The large change of the electron configurations of [Co(bpy)(3)](2+) and [Co(bpy)(3)](3+) is considered to cause the slow charge transfer and large reaction entropy change.