The potency of thermosonication as a pre-treatment technique to deactivate microorganisms was investigated in this work in order to reduce the biofouling in Reverse Osmosis (RO) system. E. coli with a concentration of 10(6) Colony Forming Unit (CFU)/mL was selected as a sample of microorganisms in water. Composite polyamide RO membrane in a stirred cell was used as a model for the membrane system. The efficiency of thermosonication in reducing the formation of biofouling on RO membrane was assessed based on two criteria i) measuring the permeate flux of the stirred cell and ii) analysing the developed biofilm on RO membrane using staining and epiflourescence microscopy techniques. Thermosonication treatment with intensity of 21.5 W/cm(2). temperature of 48 degrees C and treatment time of 4 min eliminated almost 10(3) CFU/ml. of the presented E. coli in the feed solution of RO and injured more than 10% of the surviving population. As a consequence, it was possible to recover the permeate flux of the treated feed by more than 0.1 L/m(2).hr during fouling treatment for 60 h. Moreover, the captured microphotographs of the membrane used with untreated and treated feeds revealed that the treated E. coli built sparse biofilm on the membrane, while the developed biofilm by untreated E. coil covered almost all the membrane area. (C) 2011 Elsevier B.V. All rights reserved.