The quantitative Bronsted basicity of butylamine (BuNH2) in the typical aprotic ionic liquid (IL) 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide, [C(2)mIm(+)][TFSA(-)], is reported. Potentiometric titration was performed for BuNH2 in the aprotic IL to experimentally determine the acid dissociation constant (pK(a)) for butylammonium (BuNH3+). The pK(a) value was estimated to be 16.6(1), and it was found that the value is significantly larger than that in aqueous solution (pK(a) = 10.6).