Abstract Compacted clay membranes are semi-permeable if the double layers of the clay particles overlap, thereby restricting the passage of ions. Semi-permeability is quantified by the reflection coefficient σ. In the design of clay liners for waste contaminant water, transport as a result of coupled transport is rarely taken into account. Where large salt concentration differences exist across natural or man-made clay barriers, water may be transported as a result of chemical osmosis. In a flexible wall permeameter two samples of a commercially available Wyoming Na-bentonite were subjected to a chemical gradient in order to monitor water transport and to obtain values for the reflection coefficient. In both experiments water transport by chemical osmosis was observed, and reflection coefficients of 0.003 and 0.001 were obtained, which are significantly lower than those predicted by the Fritz–Marine model and values obtained from Bolt (1982). However, the values are in good agreement with those obtained by Bresler (1973). Both experiments showed a period of 50 h of linear pressure increase as a result of chemical osmosis, after which the pressure difference dropped, i.e. after reaching a maximum pressure difference the water flow was reversed. The reversal of the water flow is consistent with diffusion osmosis, which is the transport of water as a result of the diffusion of ions in the absence of an externally applied electrical field. However, diffusion osmosis is limited to clays of low cation exchange capacity with high pore water concentrations and porosities.