An integral part of ion exchange rate models require the inclusion of a mass-transfer expression - such as that developed by Rahman and Streat [Chem. Eng. Sci. 36 (2) (1981) 293-300]. These models have been extended to numerous applications including mixed-bed ion-exchange important in ultrapure water processing for the microelectronics, power, food and pharmaceutical industries. This paper describes the extension of ion exchange rate models that can handle an arbitrary number of species with any valency, especially multi-valent dissociative species. Two solution strategies are presented and compared using three sets of water chemistry data from Nuclear power plants. (C) 2004 Elsevier B.V. All rights reserved.