The use of modern high flux reverse osmosis membranes with low salinity feeds at high recovery rates, makes the measurement of fouling by flux decline difficult in practice. Early detection of fouling is important, because the onset of fouling occurs at a critical flux which is difficult to predict a priori. An alternative method, measuring the electrical properties of the membrane is proposed here. The fouling of a reverse osmosis membrane was measured with an INPHAZE (TM) Electrical Impedance Spectroscope from frequencies of 10(-1) to 10(5) Hz. Both the conductance and impedance showed dramatic changes when the reverse osmosis membrane was fouled by a small amount of precipitated divalent salts. From these experimental results, electrical properties of the reverse osmosis membrane system were determined. This paper takes electrical properties determined in previous work and models the electrical properties of the first and second stages of a sea water desalination process. The decrease in the electrical conductivity of the membrane skin layer, which would accompany fouling, is significant at frequencies below 100 Hz. Such a method has great potential for online measurement of membrane fouling and more effective operation of membrane filtration systems.