The backflow process in reverse osmosis membranes induced by the osmotic pressure of a salt feed solution was investigated. The work was performed on an improved RO system based on 2.5 '' spiral wound module. The results showed influence of the salt concentration in, translated as osmotic pressure driving force. An important phenomena observed was that the flux through the membrane tends to decrease with the increase of salt concentration, probably due to secondary concentration polarization effects in the permeate side.