The mechanism for variable sorption capacity of boron on ion exchange resins in multicomponent systems was established. The model was derived from fundamental continuity equations and uses correlations and constants to estimate system properties and parameters as a function of time and bed depth. Implementation of solution thermodynamics for boron into the rate model was completed successfully. The decision of optimal time and distance increments is essential because of errors incoporated with numeric integration. The time and distance increments between 0.005 and 0.01 are recommended for most cases for proper run time and small total material balance errors. The model obeys the acceptable and required trends at wide operating conditions with increased complexity of the water chemistry. As the complexity of water chemistry increaesd, the run time and total material balance errors increased.