Although a laboratory test showed that up to 100 mg/L chlorine dioxide (ClO₂) at pH of about 7.5 did not degrade a polyamide (PA) reverse osmosis (RO) membrane, field testing was required before ClO₂ could be recommended for use as a biocide in RO plants. ClO₂ without any free chlorine byproduct was generated from sodium chlorite by electrochemical or catalytic methods. ClO₂ was effective in preventing biofouling and made it possible to remove existing biofilm in RO elements. At one plant, 0.3 mg/L ClO₂ was required during summer with high temperatures to prevent biofouling, but the required concentration was less at lower temperatures. The dose rate of ClO₂ was high enough when ClO₂ was detected in permeate and concentrate. Before being used in large scale, it must be determined at which conditions ClO₂ can be used without degrading PA RO membranes.