The objective of this work is to present a general methodology and to describe the modeling of ion-pairing extraction of organic acids, including amino acids, with quaternary amines, allowing for the prediction of equilibrium concentrations with different operating conditions. The model takes into account the hydration of the organic phase, thus making it also valid for any organic phase used. A rational design of the operating conditions to be used to optimize extraction and stripping is then possible from simulation studies. The model developed is tested by comparing predictions of equilibrium concentrations with experimental studies using lactate extraction with different amine concentrations. Validation is extended to the stripping process using chloride as the counterion; the model thus obtained adequately describes the extraction and stripping of lactate.