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 fouling process, where the polarisation was greatly enhanced by the formation of an unstirred layer. Using colloidal silica at a concentration of 200 ppm as model foulant and 2000 ppm NaCl as background ionic solution, and operation at a constant flux of 30 L/m(2)h, it was found that the CP level increased by 75% whereas only 22% increase in the fouling resistance (R-f) was observed. The second method, ultrasonic time domain reflectometry, was used to monitor the growth of the fouling layer. The change in amplitude of the reflected signal was correlated to the amount of silica deposited on the membrane layer. Both techniques are valuable in the study of fouling or can be applied as early warning systems to provide critical information such as the level of concentration polarisation and the extent of the fouling layer.