Solubilities of the pairs, calcium sulphate–silver sulphate and calcium sulphate–calcium iodate, were measured in various electrolyte solutions at different concentrations at 35°. The solubilities of the individual salts increase with increasing ionic strength, but the ratios [Ca²⁺]/[Ag⁺]² and [SO₄^2–]/[IO₃^–]² are found to depend only on the concentration and nature of the ions of the opposite charge and not on the ionic strength.