Environmental geotechnics involves several engineering applications (such as the salinification of groundwater and the storage of nuclear wastes) for which a deep understanding of the chemo-mechanical behaviour of soils is a fundamental component. Despite the vast number of situations in which couplings between chemistry and mechanics occur, the available experimental results are quite limited and the available constitutive models rely on limited evidence. Moreover, available experimental results are essentially focused on highly active clays. This paper presents the results of an experimental program on the chemo-mechanical behaviour of mixtures of a low activity clay (illite) and sodium chloride solutions at different concentrations. A correlation between the ionic concentrations of the pore liquid and the osmotic suction of the soil is proposed; this allows the results to be presented and discussed in terms of changes in osmotic suction and independently of the pore liquid components. The experimental programme consists of oedometric tests involving changes in the vertical stress and in the osmotic suction. The experimental results point out a correlation between the osmotic suction and initial oedometric modulus, as well as between the osmotic suction and yield stress. The experimental results are used to extend a constitutive framework for soils in order to take the observed chemo-mechanical couplings into account. The numerical model has been calibrated for the illite using the parameters obtained through tests under mechanical loading at a constant osmotic suction and validated using more elaborate stress paths. The presented experimental and constitutive investigation builds a basis for the assessment of the behaviour of low activity clays in engineering applications where pore liquid chemistry plays a major role.