The electrochemical characteristics of 9-fluorenone have been investigated in various ionic liquids as a means of determining the extent of interaction of representative Lewis acids with this ketone. The ionic liquids studied, with Lewis acids in a particular medium, are : 1-ethyl-3-methylimidazolium tetrafluoroborate (EMI BF4) [BF3 etherate], 1-butyl-1-methylpyrrolidinium trifluoromethanesulfonate (BMPY TfO) [Hf(TfO)(4) and Sc(TfO)(3)], and aluminum chloride : 1-ethyl-3-methylimidazolium chloride (AlCl3 : EMICl) [aluminum chloride]. In EMI BF4 and BMPY TfO, 9-fluorenone undergoes two successive one-electron transfers; however, addition of a Lewis acid produces a new reduction process shifted to a more positive potential. The extent of this shift is taken as an indication of the strength of the Lewis acid interaction with 9-fluorenone. The strongest Lewis acid : 9-fluorenone interaction, judging from the magnitude of the potential shift, is found in the AlCl3 : EMIC ionic liquid, followed by that of Hf(TfO)(4) in BMPY TfO. The interaction of Sc(TfO)(3) with 9-fluorenone in BMPY TfO is considerably less than that of Hf(TfO)(4) In EMI BF4, however, Sc(TfO)(3) interacts quite strongly with 9-fluorenone, indicating that the triflate anion interacts extensively enough with Sc3+ in BMPY TfO to restrict its complexation of 9-fluorenone. (C) 2013 The Electrochemical Society.