The transport processes of divalent cations in the xylem have been studied using semiconductor radiation detectors, operated at room temperature. Cut shoots of a papyrus plant (Cyperus papyrus spec.) have been used as a model system. Using a sequential supply of labelled and unlabelled solutions (45Ca,115Cd) in the presence or absence of a chelating agent (EDTA), the CEC of the transport system has been determined. Results have been verified in perfusion experiments, using isolated stem segments. Values obtained for CEC, expressed in moles calcium or cadmium per unit of vessel wall surface, range from 3 to 5 x 10∼2. Such a high density (1.8 to 3 ions A0–2) is not realistic, and shows that the exchange sites are located in an extended volume rather than only at the surface of the vessel wall. Different possible consequences regarding the cation exchange processes and calcium transport are discussed.