Base exchange. The reversible chemical reaction in which certain ions in a crystal lattice on particle surfaces or edges can become [[hydrate|hydrated]] in the presence of aqueous solutions with the result that they can be replaced, equivalent for equivalent, by ions (usually cations) exhibiting similar charge from the solution without a loss in the crystal structure.

According to a widely held view, the cations do not occupy precise sites: some are located in a plane parallel to and within a few angstroms of the clay surface (Helmholtz layer); others are contained in a zone of diminishing concentration (called the [[diffuse layer]]) extending outward from the Helmholtz layer.

Materials, particularly clays, which exhibit this property support [[surface conductance]] with their [[exchange cation|exchangeable cations]].