Abstract Two non-invasive techniques were demonstrated to detect silica fouling in reverse osmosis. The first technique,the sodium chloride tracer test, enabled the estimation of concentration polarisation (CP) level during the foulingprocess, where the polarisation was greatly enhanced by the formation of an unstirred layer. Using colloidal silicaat a concentration of 200 ppm as model foulant and 2000 ppm NaCl as background ionic solution, and operation ata constant flux of 30 L/m 2h, it was found that the CP level increased by 75% whereas only 22% increase in thefouling resistance ( R f ) was observed. The second method, ultrasonic time domain reflectometry, was used to monitorthe growth of the fouling layer. The change in amplitude of the reflected signal was correlated to the amount ofsilica deposited on the membrane layer. Both techniques are valuable in the study of fouling or can be applied asearly warning systems to provide critical information such as the level of concentration polarisation and the extentof the fouling layer.