The effects of additives, coumarin and saccharin, on electrodeposition of nickel were investigated in a hydrophobic room-temperature ionic liquid, 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide (BMPTFSA) containing Ni(TFSA)(2). Although the UV-vis spectra showed that the coordination environment of Ni(II) was not affected by addition of coumarin or saccharin, the overpotentials for the Ni(II)/Ni reduction increased and the current density decreased. The diffusion coefficients of the Ni(II) species in the presence of both additives were estimated to be close to that in the base electrolyte from the chronoamperometric results. The nucleation/growth model of Ni was changed from the instantaneous type to the progressive one after addition of coumarin or saccharin. The effects of these two additives on the morphology of the Ni deposits in Ni(TESA)(2)/BMPTESA were also characterized. (C) 2014 Elsevier Ltd. All rights reserved.