Electrical Impedance Spectroscopy (EIS) was explored as an in situ tool for the detection of fouling of reverse osmosis membranes, during filtration of industrial (molasses) wastewater and model feeds containing silica and BSA foulants. The electrical capacitance measured at low frequencies (< 100 Hz) was found to be the most sensitive electrical parameter for detecting fouling, during the early stage of flux decline. The effective gain (the percentage of change in capacitance divided by the percentage change in the flux) was 3 for molasses waste water, IA for silica and 2.5 for BSA. The formation of the reversible fouling layer occurred in conjunction with concentration polarization during this period and EIS was more sensitive than flux decline in monitoring this process. Hence [IS has potential in industrial application for fouling detection. Further the impedance spectra after cleaning were consistent with the flux recovei:y. The impedance spectra obtained following macromolecular (BSA and molasses wastewater) fouling was found to be significantly different from those obtained with colloidal (silica) fouling. The decrease in capacitance observed for BSA and molasses wastewater, suggests the formation of a gel like layer on the membrane surface, while the increase in capacitance of the silica suggests a loose cake which enhances concentration polarization. Hence EIS may also be capable of indicating the type of fouling which is occurring at the membrane surface. (C) 2015 Elsevier EV. All rights reserved