Electrical potentials in cell walls ( Wall ) and at plasma membrane surfaces ( PM ) are determinants of ion activities in these phases. The PM plays a demonstrated role in ion uptake and intoxication, but a comprehensive electrostatic theory of plant-ion interactions will require further understanding of Wall . Wall from potato (Solanum tuberosum) tubers and wheat (Triticum aestivum) roots was monitored in response to ionic changes by placing glass microelectrodes against cell surfaces. Cations reduced the negativity of Wall with effectiveness in the order AlThis order resembles substantially the order of plant-root intoxicating effectiveness and indicates a role for both ion charge and size. Our measurements were combined with the few published measurements of Wall , and all were considered in terms of a model composed of Donnan theory and ion binding. Measured and model-computed values for Wall were in close agreement, usually, and we consider Wall to be at least proportional to the actual Donnan potentials. Wall and PM display similar trends in their responses to ionic solutes, but ions appear to bind more strongly to plasma membrane sites than to readily accessible cell wall sites. Wall is involved in swelling and extension capabilities of the cell wall lattice and thus may play a role in pectin bonding, texture, and intercellular adhesion.The cell wall (CW) is composed of various crosslinked units (macrofibrils, microfibrils, micelles, cellulose units, and linked agents such as neutral sugars, pectin, proteins, and ions; Cosgrove, 1997; Buchanan et al., 2000). The CW behaves as an ion exchanger where the fixed CW charges interact with exchangeable ions in the surrounding solution (Briggs and Robertson, 1957; Gillet and Lefèbvre, 1981; Sentenac and Grignon, 1981; Irwin et al., 1985; Richter and Dainty, 1989a, 1989b, 1990a, 1990b Grignon and Sentenac, 1991). The net CW charge is negative and results from weakly dissociating acidic groups having pK a values similar to those of polyGalUA, the principal origin of the negative charges (Ritchie and Larkum, 1982; Saftner and Raschke, 1981; Richter and Dainty, 1989a; Buchanan et al., 2000). Some positive charges occur too, mainly associated with CW proteins (Cassab and Varner, 1988; Buchanan et al., 2000).The CW determines cell dimensions (Taiz, 1984) and intracellular volume. The volume of the CW is a consequence of the dimensions of its internal spaces, i.e. the distances between the intra-CW units (Shomer et al., 1984; Shomer and Levy, 1988), which are determined by the repulsive strengths of diffuse double layers (Shomer et al., 1991). The swelling of parenchyma CW is restrained by the increase of valence and concentration of the exchangeable ions and with the decrease of the dielectric constant of the bulk solution (Shomer et al., 1991; Shomer, 1995). Although cell expansion and CW extension have been studied comprehensively (Cosgrove, 1997), the properties governing the volume of the CW, from the point of view of the dimensions of its internal spaces...