Mass action quotients for the synthetic zeolites Linde 4AXW, 13X, AW-300, AW-400, AW-500, Norton Zeolon and the natural zeolites clinoptilolite and erionite were determined for low cesium or strontium concentrations in the binary systems cesium-sodium, cesium-
potassium, cesium-rubidium, strontium-calcium and strontium-sodium.