The adsorption of dodecyl sulphate on barium sulphate has been measured as a function of concentration. Comparison with krypton adsorption and stearic acid adsorption has shown that an amount of dodecyl sulphate equivalent to two layers was adsorbed at saturation, in agreement with some, but not all, previous reports for such systems.�The possibility that precipitation of barium dodecyl sulphate may have contributed to the apparent adsorption has been specially investigated and the effect was shown to be small. Indirect evidence in support of two-layer adsorption has also been found from sedimentation studies. The adsorption of a second layer, with hydrophilic groups oriented away from the solid, provides the most satisfactory explanation of rewetting at high concentrations of adsorbate.