Acid-base equilibria of the aniline derivatives in n-BuOH were investigated. The over-all dissociation constant (K^n-BuOH_BH=K^O_BH/K_BK_BHX=[H⁺] [B] f_H/[BH⁺] f_BH) and the dissociation constant of ion pair (K_BHX=[BH⁺] [X^-] f_BHf_X/[BH⁺X^-]) were estimated. The ion pair (BH⁺X^-) existed to a considerable extent. The values of pK^n-BuOH_BH and pK_BHX were respectively : 3.47, 2.83 for 4-methyl-3-nitroaniline ; 3.33, 2.66 for 4-carbethoxyaniline ; 2.94, 2.42 for 2-methoxy-5-nitroaniline ; 2.84, 2.74 for m-nitroaniline ; 2.79, 2.84 for 2-methyl-5-nitroaniline. The basicity of the anilines in n-BuOH increased as compared with that in water (ΔpK=pK^n-BuOH_BH-pK_a=0.4-0.8).