Chemically active saturated clays are considered in a two-phase framework. Two situations are delineated: homoionic clays and heteroionic clays. In homoionic clays, the solid phase contains clay particles, absorbed water and a single salt. The fluid phase, or pore water, contains free water and salt. Water, and possibly salt, can transfer between the two phases. In addition, part of both species diffuse through the porous medium. A global understanding of all phenomena, deformation, mass transfer and diffusion, is provided. Emphasis is laid on the chemo-mechanical constitutive equations in an elastic-plastic setting. In heteroionic clays, the presence of several cations in the clay clusters requires modifications with respect to this framework: then the electrolytic nature of pore water has to be accounted for.