Extraction selectivity and its dependence on pH were studied in binary mixtures of lactic, glutaric, malic, and maleic acids using extractants containing Primene JMT. Selectivity was found to depend strongly on pH. The effect of pH was analyzed through an analogy to the addition of a strong base to systems containing the species formed in the organic phase, whereby the added base reacts first with the strongest acid present. The study shows that the stronger acid is selectively extracted through ion-pair formation in cases where the extractant is a stronger base than the anions of both acids and the hydrophobicities of the two acids are about the same. However, the weaker acid is preferably extracted by H-bond interactions with the formed ion-pair in the above-stoichiometric range. Upon pH elevation, the selectivity to the stronger acid decreases in the above-stoichiometric range but increases in the range where Z < 1. For the range below molar stoichiometric extraction, a "double salt" is formed and converted into a divalent ion pair, while one-half of the acid is transferred to the aqueous phase. However, anion exchange with the other acid's anion can take place and lead to transfer of the other acid into the aqueous phase.