The occurrence of significant co-extraction of buffer anions by the ion exchanger Aliquat 336 is unavoidable when high levels of system buffering is required. The co-extraction will result in inaccurate equilibrium and mass-transfer characterization of such a system unless its occurrence is taken into account, making process design and control difficult. A study of the equilibrium of phenylalanine extraction using Aliquat 336, a system where high levels of hydroxyl co-extraction occurs, was used as a model case to develop a method of accounting for co-extraction in mass-transfer modeling. Analysis of the equilibrium between bulk-aqueous-phase chloride and phenylalanine concentrations during mass transfer in a stirred-transfer cell showed there to be linear equilibrium relationships between the two parameters for a given extraction system of the form C Cl,t = β£(C A,t -C A,0 ) for forward extraction and C Cl,t = C A,t + C Cl,0 for backward extraction. The constants of proportionality of these relationships, or the "co-extraction constants," β£ and , were shown to be related to the selectivity of Aliquat 336 for the phenylalanine anion by the relationships β£ = -(1/S + 1) and = -(1/S -1 + 1). The linear equilibrium relationships were used to develop two-film theory mass-transfer models for both forward and backward extraction that account for co-extraction. These showed much higher accuracy in modeling stirredtransfer-cell data than the equivalent models which ignored co-extraction.