AbstractPurified xylem cell walls were prepared from isolated xylem bundles of tomato (an inbred line ofLycopersicon esculentumMill, cv. Tiny Tim). Adsorption and exchange experiments were carried out with115Cd2+,82Rb+and82Br−. The application of γ‐ray spectroscopy permitted the simultaneous measurement of several ions applied together. The cell‐wall water volume was shown to be independent of the external pH and solution ionic strength, possibly due to the presence of lignin. The Donnan Free Space (DFS) volume could be determined as a constant 0.15 dm3per kg cell‐wall dry weight. Consequently, the total cell‐wall cation exchange capacity (CEC) could be estimated based on the DFS volume, and amounted to approximately 1000 mol m−3negative charges. The results of Cd2+‐Rb+exchange experiments indicated an apparent CEC value of about 350–450 mol m−3DFS, at external pH ∼ 4. These data are in agreement with earlier reports on xylem wall CEC, and indicate the weak acid characteristics of the charge groups. The rational selectivity coefficient RCdRb, of the cell wall was shown to depend on external ion fractions and ionic strength, with a maximum RCdRbof 450 at ionic Cd2+fraction near 0.3, based at the smallest experimental ionic strength of the external solution. The adsorption of Cd2+, applied at relatively high concentrations, was shown to be stimulated by simultaneous application of hi