Abstract One of the available methods for the control of scale deposition in water desalination processes is the addition of such compounds as polyphosphates in trace quantities. More recently, synthetic organic polyelectrolytes have been used in this connection. The mechanism by which these inhibitors act has not been extensively studied. A study has been made of the effects of a wide variety of polyelectrolyte additives on the nucleation and crystal growth of calcium sulphate dihydrate from supersaturated aqueous solution. The results show that the degree of inhibition at a given supersaturation depends upon the chemical nature of the additive, the concentration and the molecular weight of the additive, and, if the additive contains ionisable groups, the pH of the crystallizing solution.