Aqueous suspensions of kaolinite from Mt. Egerton, Victoria, have been leached at pH 2.25 and 6.0. More ions, in particular aluminium, than predicted from the published cation-exchange capacity have been detected in the leachate. An activation energy of 5 kcal/mole has been calculated for this process of surface leaching. Using a supporting electrolyte of, for example, 10-4M KCl, the electrophoretic mobility of leached and raw clay has been examined as a function of pH, and differences in mobility have been correlated with aluminium removal from the clay surface.