Ion exchange is one of the powerful work horses in the recovery and purification of biotechnological products, such as antibiotics, amino acids, and proteins. Rational design of bioseparation processes involving ion exchange requires the reliable description of governing equilibria. The conventional description of multicomponent ion exchange equilibria [1] is based on stoichiometric, constant selectivity models, and complete exclusion of co-ions from the resin matrix. Also, uptake mechanisms, other than ion exchange, are usually not accounted for explicitly. This general problem is illustrated for the uptake equilibria of aqueous acetic acid/acetate solutions onto strong anion exchange resins in Figure 11.1. The parameter Q is the ion exchange capacity of the resin (also called the density of fixed charges on the ion exchange matrix), CT is the anion concentration in the liquid phase, ∆Φ the difference in electric potential between the liquid and resin phase, and π is the osmostic effect (to be explained later).