Efficient operation of the reverse osmosis (RO) process for desalination of brackish water requires the reduction of silica in feed water to safe levels in order to avoid scale fori-nation. After extensive jar testing, two pilot plants which represent the chemical clarification media filtration RO pretreatment processes at two major treatment plants in Saudi Arabia were constructed and operated on selected variable chemical combinations and dosing rates. The aim was to reduce silica concentration from the two deep well groundwater sources, of different silica content, to the acceptable levels of RO feed water (silica removal >23% for one source and >48% for the other source). For the low silica water source, the dose combination of lime (110 mg/L), soda ash (220 mg/L) and alum (30 mg/L) was adequate for silica control, while using caustic soda alone required a dosage off 25 mg/L. for the same level of silica removal. F or the high silica water source, the dose combination of lime (90 mg/L),soda ash (300 mg/L), sodium aluminate (25 mg/L) and anionic polymer (0.05 mg/L)was adequate for silica control: a dosage of 175 mg/L. caustic soda alone was adequate for the same level of silica removal. The use of caustic soda (NaOH) as a single chemical was found a viable alternative to the lime-soda precipitation aid process.