A simple and reliable technique for on line detection and diagnosis of scaling and fouling deposits could improve significantly operational control of RO plants. This paper presents results of an ongoing research project aiming to develop a novel method for online detection of membrane fouling by a residence time distribution (RTD) technique. The method is based on determination of flow dispersion intensities from online RTD signals that can be simply measured. The systems investigated in this paper are spiral wound membranes fouled with either Mg(OH)(2) or CaCO3. Analyses of RTD data of fouled membranes show that an increasing membrane permeability loss is accompanied by a systematic increase in the dispersion coefficient, thus providing an indicator for detecting a fouling event. It is also found that the magnitude of the dispersion coefficient is affected by deposit morphology, thus indicating the possibility for diagnosing the nature of the fouling deposit. Another fouling indicator may be derived from the observation that the extent of the dead volume monitored by the RTD signal increases with the fouling level while dead volumes are absent in clean modules.