The successful operation of reverse osmosis (RO) systems is often impaired by fouling on and failure of the membranes. To optimise membrane operation there is a need for tools and methods to investigate the root causes of membrane fouling and failure. The present study describes the use of light microscopy and electron microscopy as tools for the analysis of fouled or damaged membranes. RO membranes are usually microscopically smooth, with roughness of 10 to 90 nm. However, light microscopy and environmental scanning electron microscopy (SEM) on membranes used at high pressure showed a high roughness (around 20 mu m), with deep, close-spaced valleys and ridges, which were caused by imprints of permeate spacers. The valleys created in the membrane accelerate and increase membrane fouling and hamper membrane cleaning. Moreover, the ridges were found to be the starting point for cracks in the membrane. SEM analysis of many used RO membranes showed large particles on the surface, in cases up to 1 mm in size. Sharp, irregular particles were observed, which had been pressed deep into the membrane surface. Moreover, small leaks and in some cases fissures up to 0.2 mm long were seen in the membrane which were obviously caused by the particles on the surface.