Binary and multicomponent exchange equilibria of the anions of Penicillin G (Pen G), 6-aminopenicillanic acid (6-APA), and phenylacetic acid (PhAc) versus Cl- on Amberlite IRA400 resins have been measured, using batch experiments in an aqueous system. When transformed into equivalent ionic fractions, the binary exchange data for the anions of 6-APA and PhAc versus Cl- could be correlated with a constant selectivity model. The binary exchange data of the anion of Pen G versus Cl- showed a decreasing selectivity with increasing equivalent ionic fraction of the anion of Pen G in the liquid phase, which is common for bulky ions. The measured data are correlated satisfactorily with the Myers and Byington model. Using parameters from correlating these binary data, the Myers and Byington model predicts the multicomponent anion equilibria with an average deviation of 32%.