The electrode reaction of tris(2,2'-bipyridine) cobalt complex, [Co(bpy)(3)](3+/2+), was investigated in 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl) amide (BMPTFSA) and 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIBF4) ionic liquids. The diffusion of [Co(bpy)(3)](2+) and [Co(bpy)(3)](3+) was affected by the viscosity and the charge densities of the complexes and the anion of ionic liquid. The difference in 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. (C) The Author(s) 2017. Published by ECS. All rights reserved.