Polarity of a series of ionic liquids (ILs) prepared by neutralization of N-alkylimidazoles and acids was measured with solvatochromic dye, Nile Red. Polarity of the N-alkylimidazolium salt-type ILs was in the comparable range to that of N,N'-dialkylimidazolium salt-type ILs. In the range, higher polarity was found when stronger acid (small pK(a) value in water) was used. This is the first report on the polarity of ILs containing dissociable protons.