Because of the versatility of reverse osmosis for removing a wide range of contaminants, U.S. EPA (Drinking Water Research Division) has been conducting laboratory and field studies to determine its effectiveness on specific inorganic and organic contaminants of concern to the water supply industry. Laboratory and field studies for the removal of specific inorganic contaminants have shown that most of the contaminants listed in the EPA drinking water regulations are highly rejected (greater than 90%) while a few contaminants, such as fluoride, nitrate, and arsenic III, are only moderately rejected. Also, tests with different membranes showed only minor differences in removals. Laboratory studies using cellulose and nylon amide membranes have shown less than 25% rejection of volatile organic compounds. A thin film composite membrane, on the other hand, looked very encouraging in short duration (less than 4 hours) tests for removing VOCs. However, when the thin film composite membrane was exposed to contaminated ground water for a longer term, certain VOCs, primarily unsaturated and aeromatic compounds, permeated the membranes within 12 to 16 hours. Thus, the future for RO in controlling VOCs is uncertain.