Data are presented which show a mathematical model based on the law of mass action to be applicable to the description of strontium removal from solutions of low ionic strength by activated alumina. Using radiochemical techniques, the concentration of strontium and alumina, the presence of competing cations, total ionic strength, and pH are shown to be the parameters controlling sorption. The use of activated alumina for removing strontium from natural waters indicates that greater than 90 per cent removal can be obtained when the concentration of calcium and strontium is within the range of concentrations normally found in water used for human consumption.