We report on the acid-base reaction of an amine solute in an aprotic ionic liquid from a structural point of view. Thus, the solvation structures of n-butylamine and n-butylammonium (BuNH2 and BuNH3+, respectively, acid-base reaction: BuNH2 + H+ ⇄ BuNH3+) in 1-methyl-3-ethylimidazolium bis(trifluoromethanesulfonyl)amide ([C2mIm][TFSA]) were investigated by high-energy X-ray total scattering combined with molecular dynamics simulations. We found that the solvation structure drastically changed as a result of the protonation reaction in [C2mIm][TFSA]. The NH3+ group was preferentially solvated by TFSA- anions in the protonated BuNH3+ system, whereas the neutral NH2 group was surrounded by both TFSA- and C2mIm+ ions in the BuNH2 system. With regard to the nearest neighbor solute-TFSA interaction, the solvation number for TFSA- anions (nTFSA) increased upon protonation (i.e. BuNH2:nTFSA = 2 and BuNH3+:nTFSA = 3). Both BuNH2 and BuNH3+ interacted with O atoms within TFSA- through hydrogen bonding (i.e. N-HO) interactions, and the N-HO distance was appreciably shorter for positively charged BuNH3+ as compared to neutral BuNH2. These results revealed that the solvation is more stable in energy for the protonated BuNH3+ group because of its stronger hydrogen bonding as compared to neutral BuNH2. This is the origin of the large Gibbs energy for the protonation reaction (ΔG = -94.7 kJ mol-1) and the high acid dissociation constant (pKa = 16.6) of BuNH2 in [C2mIm][TFSA] solution.