Equilibrium models are typically used to describe ion-exchange-resin regeneration. These models are accurate for most applications due to the high concentration of regenerant used. However, for ultrapure water applications, where regeneration efficiencies must exceed 99.9% (sites in the hydrogen or hydroxide form), equilibrium models underpredict the total chemical contact time required. One approach to correct this modeling deficiency is to replace the equilibrium assumption with a kinetic expression. This paper describes kinetic modeling of ion-exchange-resin regeneration using both the Nernst-Planck and Fick’s Law approaches. The model compares favorably with literature data.