An accurate expression for ionic mass-transfer coefficients in ultrapure water is essential for detailed modeling and simulation of ion exchange. This paper presents an evaluation of 11 dimensionless mass-transfer correlations. Ten sets of experimental data, from both our laboratory and the literature, are used to define the best expression for ultrapure water applications. The correlation of Dwivedi and Upadhyay (Ind. Eng. Chem. Process Des. Dev. 1977, 16 (2), 157) gives the best results.