Desalination plays an important role in producing pure water from brackish water. Reverse osmosis (RO) is by far the most efficient way to remove colloidal and dissolved silica, which can be found in high concentrations in brackish water. The presence of silica and its ability to foul membranes limits the use of silica bearing waters for desalination and when used, it has many economic penalties. This study examines the effect of silica polymerisation in the presence of polyvalent cations and anions in RO systems. Source of silica in the experimentation was from commercial grade sodium metasilicate (Na2O3Si.9H(2)O). Tbe membranes used were polyamide and thin film manufactured by Osmonics. Use of glassware is minimised to avoid the possibility of any contribution by silica leaching into solution. The feed solution consists of silica, calcium and magnesium ions in various concentrations to determine the effect of polyvalent ions on polymerisation and the appropriate pre-treatment technology. The experiment was set up in a way as to simulate the conditions that would be encountered in a desalination plant. Concentration polarisation (C/P) in the system was experimentally determined with a simple technique that was developed and its effects on fouling are examined. In addition tests were carried out to examine the actual fouling mechanism in reverse osmosis units under various experimental conditions. Furthermore the effect of cleaning, with distilled water and with pulsations as well as with commercially available cleaners were examined. Some commercially available cleaners were capable at partially restoring the flux. Further investigation is underway to examine the effectiveness of new silica specific inhibitors.