Anti-scalants, which are widely used for scale control in desalination processes, are essentially impurities capable of inhibiting scale precipitation and weakening scale adhesion on a flow surface. Many basic studies indicate that metallic ion impurities, notably Zn ions, can hinder significantly CaCO3 precipitation from hard waters and alter crystal morphology. Recent studies demonstrate a substantial potential for suppressing CaCO3 scale formation in hard water heating systems by trace concentrations of Zn ions. The objective of the present work was to explore the possibility of controlling CaCO3 scale formation in RO membrane systems by a similar technique. Results obtained with a tubular RO system are most promising. A Zn++ concentration of the order of 2 PPM was able to exert a marked suppression effect on both bulk precipitation of CaCO3 and on membrane scaling on waters of moderate hardness.