A transient model of down-flow through an ion-exchange column in which the resin swells has been developed. The model is herein described and results are presented. Wall friction can lead to high bed stresses when the resin in columns with high length to diameter ratios swells. These stresses can lead to high and potentially excursive hydraulic pressure drops along a column. A non-dimensional grouping that effectively correlates the final steady-state hydraulic behavior of a column, with the resin compressibility and column geometric and flow parameters, has been determined. Published by Elsevier Ltd.