Fouling of membranes by organic substances precludes that these organics are present in the feed water, most likely in a dissolved state. Analysis of feed waters for such constituents can help to find the causes, and later the solutions, for membrane fouling. Two examples are presented in which this strategy was successful, in the first example polysaccharides and hydrophobic organic compounds were responsible for membrane fouling, in the second example the culprit was an anionic polymeric flocculant aid.