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The influence of pH and of added lattice and certain non-lattice ions has been studied towards the instantaneous sorption of tracer 51 CrO 4 2– species on ignited lead sulphate and barium as carrier surfaces. As expected, an increase in the carrier capacity is observed with increasing pH and lattice cation concentration. However, non-lattice cations (Ba 2+ and Sr 2+ ions) with a similar charge and ionic dimensions exhibit abnormal behaviour on the surface of PbSO 4 , and Sr 2+ ions behave in a similar fashion on the surface of BaSO 4 . This adverse effect can be overcome by increasing the pH or lattice cation concentration in the suspension medium. An attempt has been made to explain this abnormality as arising from masking of the active adsorption centres on PbSO 4 and BaSO 4 surfaces under the experimental conditions employed.