In the absence of impervious natural soils, compacted mixtures of bentonite and sand have been used to form barriers to fluids, and the focus of this paper is on the hydraulic conductivity behaviour of these compacted mixtures. Results of laboratory investigations are presented to show the influences of bentonite–sand content, compaction water content, system chemistry, and changes of system chemistry. Some of the findings are as follows: (i) low hydraulic conductivity requires continuity of the bentonite matrix within the mixture, and this in turn requires both adequate bentonite content and adequate bentonite distribution (i.e., mixing); (ii) in well-compacted mixtures containing up to 20% bentonite in dry mass, sand forms the load-supporting framework and gives the mixtures dimensional stability at the macro level (crack resistance); and (iii) the hydraulic conductivity of mixtures containing freshwater, high-swell bentonite, when permeated with a strong saline solution, increased by only small amounts, suggesting that the fabric of bentonite enclosed within the sand framework was little influenced by this change of system chemistry. Key words : mixture, bentonite, sand, permeability, laboratory study.