Reverse osmosis (RO) is an important membrane separation process, widely used for desalination applications RO is part of a rapidly growing market as the demand for clean fresh water around the world continues to expand A major industrial challenge for RO operations is control of fouling of the membrane modules Fouling decreases production capacity and water quality and increases operating costs. Biofilm growth in the membrane modules, commonly referred to as biofouling, is in practice arguably the major fouling type Different cleaning strategies are employed to remove such foulants and evaluation of cleaning effectiveness is often difficult, with operators relying on indirect measurements of fouling such as the pressure drop across the membrane module The present study aims to evaluate chemical cleaning of biofouled RO membranes using magnetic resonance imaging (MRI) Membrane fouling simulators (MFS) were fouled in the laboratory, then subsequently cleaned using combinations of sodium dodecyl sulphate (SDS) and sodium hydroxide (NaOH) and observed using MRI. Both MRI structural and velocity images showed marked changes in biofilm distribution A small volume of accumulated biomass had a large Impact on the effective surface area for water production, the value of which was more accurately calculated using the velocity images. The extracted effective membrane surface area correlated well with the feed channel pressure drop Additionally with this in situ MRI technique, the effect of fouling extent and time on cleaning effectiveness (biomass removal and effective surface area) were investigated. Cleanings at an early stage of biofouling was more efficient in removing biomass than cleaning performed at a later stage (C) 2010 Elsevier B.V All rights reserved.