The concentration of an acid's anion in the aqueous phase affects that acid's extraction efficiency by amine-based extractants. The anion effect is explained by using the acid-dissociation equation in the case that the acid is mainly extracted though H-bonding or solvation. This explanation does not hold in the extraction of strong acids such as hydrochloric or dichloroacetic acid by a relatively strong base extractant (pK(aB) much greater than pK(aA)). To explain the anion effect in these systems, we reexamined our theory and modified it with the Gibbs-Donnan model. The modified theory shows that the proton activity in the organic phase could differ significantly from that in the aqueous phase, depending on the relative activities of the anions in those two phases. Thus, in turn, the relative activity of the anion affects the protonation of the amine in the organic phase, pK(aB). Accordingly, the modified theory successfully explains the effect of the anion concentration on the extraction efficiency.