Boom Clay and Ypresian Clays have been considered as potential geological host formations for radioactive waste disposal in Belgium. Considering the significant differences in pore water chemical composition between several sites involving these two formations as well as the possible evolution of the chemical composition during the large lifespan of a radioactive waste disposal, it appeared important to investigate the effects of porewater chemical composition on the hydro-mechanical behaviors of these two potential host formations. In this study, these effects were investigated by carrying out specific oedometer tests. Different compositions were considered for this purpose: distilled water, synthetic site water, and Sodium Chloride solutions at concentrations of 15 and 30 g/L. Clear effects of pore water chemical composition on the hydro-mechanical behavior were observed. Furthermore, the pore water chemical composition effects were found to be mineralogy, stress state and salt concentration dependent. The competition between the physico-chemical and mechanical effects was identified: the porewater chemical composition effect is clear only in the low-stress range where the hydro-mechanical behavior is dominated by the physico-chemical effect.