Laboratory tests with simulated waters in the range of those in a prospective desalination plant were carried out to determine the water quality effects on silica precipitation both in batch and dynamic tests using RO membranes. In this study the effect of cations on silica polymerization was investigated. Previous investigations established that the best operating pH range is below 6.5, which precludes polymerization as well as precipitation of silicates. Thus this pH was adopted for this study. Results indicated that magnesium hardness was more effective than Ca hardness in silica polymerization rate. At a given hardness, decreasing the ratio of calcium to magnesium was found to increase the polymerization rate. This was apparent in batch tests and during the initial period of dynamic tests. Also batch tests showed that polymerization increases with the degree of supersaturation. However, in dynamic tests the flux decline was higher at low feed concentrations and higher Ca:Mg ratio after a period of operation. This inflection on the effect of Ca:Mg ratio and silica concentration in dynamic tests is possibly due to precipitation of monomeric silica and deposition at the surface resulting in a less permeable scale. While at high concentrations or when polymerization is rapid (at high Ca:Mg) bulk polymerization results and deposits are of more porous colloidal silica. Membranes, retentate and batch precipitates were analyzed using electromagnetic dispersion spectroscopy and an electron microscope. It was found that the batch precipitates consisted only of silica and oxygen, whereas those of dynamic tests had minor amounts of Ca and Mg in addition to many contaminants, most notably iron which was possibly due to corrosion in the pump.