An effective stress concept for active clays is formulated to improve the modelling of these materials under a variety of environmental conditions. In particular, the mechanical effects induced by a change in the chemical potential of pore water associated with a change of its pressure (even in the negative range), as well as a change of its chemical composition, are considered. Some implications of the proposed framework are shown with reference to shear strength results related to unsaturated conditions. The proposed approach allows to describe the shear strength envelope consistently by using a unique set of shear strength parameters irrespective of the pore water chemical composition and of the saturation state.