The electrodeposition of cobalt was investigated in an ionic liquid, 1-n-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl) imide. The overpotential for the electrodeposition of Co was reduced remarkably by elevating the temperature up to 200 degrees C, probably due to the change in the coordination environment of Co species. The addition of acetone led to the decrease in the overpotential, indicating the selective coordination of Co cation with acetone enhanced the charge transfer rate. It has been suggested that the charge transfer and crystal growth in the ionic liquid are affected by the coordination environment of the dissolved metal species and the structure of electric double layer. (c) 2007 The Electrochemical Society.