AbstractThe solubility diagram of the quinary system Na+‐Mg++‐1‐anminonaphtalene‐6‐sulfonate‐1‐aminonaphtalene‐7‐sulfonate‐Cl−‐H2O has been partially investigated between 0° and 75°.From this diagram a simple and effective procedure to separate the two CLEVE'Sacids has been calculated, with yields in acid 1.7 (extracted as sodium salt) of 95.1% and in acid 1.6 (obtained as magnesium salt) of 70.0%.This theoretical procedure was adapted to the fraction of a crude industrial