The invention relates to a process to produce brine of improved purity wherein a salt source, contaminated with sulphate ions and alkaline (earth) metal ions, is dissolved in water in the presence of an effective amount of a retarding agent comprising at least one low and at least one high molecular weight retarding agent. Said retarding agent effectively reduces the level of the alkaline (earth) metal sulphate dissolved in the brine, particularly the amount of calcium and sulphate. Preferably, the combination of the low and high molecular weight retarding agents is synergetic in the reduction of dissolved alkaline-earth metal sulphate.