Novel low pressure reverse osmosis and nanofiltration membranes and a process for their preparation are disclosed. Polyamide reverse osmosis membranes are contacted with organic sulfonic acid solutions without the need for additional treatment by a rejection enhancing agent. These membranes provide sodium chloride rejections of greater than 20 percent and water fluxes greater than 15 gallons per square foot per day at a test pressure of 75 psi. Optimally treated membranes when tested similarly on 0.2 percent magnesium sulfate provide rejections greater than 95 percent with water fluxes greater than 15 gallons per square foot per day.