This paper describes a sodium chloride tracer response technique, to determine the effect of fouling on concentration polarization level in a reverse osmosis (RO) system operated in the constant flux mode. Colloidal silica and alginic acid were used as model fouling agents. It was found that the formation of the 'unstirred' fouling layer greatly exacerbated the concentration polarization (CP) level in RO separations, with a more pronounced effect at high flux operation. The results suggested that operation at high flux could significantly reduce the apparent permeability, but that this is through a dramatic increase in CP and consequent loss in driving force via the enhanced osmotic pressure rather than through an additional hydraulic resistance. The study also shows that the sodium chloride tracer response technique is a promising technique that can be applied as a non-invasive tool to detect and monitor fouling development in reverse osmosis. (c) 2006 Elsevier B.V. All rights reserved.