The total volume change included gel and surrounded water with the swelling of five types of spherical cross-linked dextran (Sephadex) that are either nonionic (G) or possess one of four different ionic groups in the same molecular skeleton : CM : sodium carboxymethyl SP : sodium sulphopropyl, DEAE : diethylaminoethyl chloride, and QAE : diethyl-(2-hydroxypropyl) aminoethyl chloride. All have been studied by dilatometry. The total volume decreased with the swelling of all Sephadexes. The maximum changes of total volume with the swelling in water were all negatives of CM, G, SP, (DEAE, QAE), which decrease in that order. These values did not depend on the concentration of sodium chloride and were closely related to the maximum heats of swelling. Total volume changes occurred by the dissociation of the ionic group of dextran derivatives that subtracted the total volume change due to hydration of Sephadex skeleton from the total volume change : -21.6 +/- 3.1 mu L mmol(-1) for CM, -2.8 +/- 8.0 mu L mmol(-1) for SP, +13.2 +/- 4.4 mu L mmol(-1) for DEAE, and +15.4 +/- 4.6 mu L mmol(-1) for QAE. These values are assumed to he reflected in the quantities of ions-water interaction of Sephadexes.