Cork processing wastewaters are very complex mixtures of vegetal extracts and have, among other natural compounds. a very high content or phenolic/tannic colloidal matter, that is responsible for severe problems of membrane fouling and drastic Flux decline. In the present work the identification and quantification of phenolic/tannic compounds, responsible for membrane fouling, is carried out through the use of reverse phase high pressure liquid chromatography (HPLC). The compounds studied were gallic acid, protocatechuic acid, vanillic acid, syringic acid, ferulic acid and ellagic acid. The very different characteristics of these compounds in terms of hydrophilicity/hydrophobicity were correlated to their rejection coefficients for two cellulose acetate ultrafiltration membranes of 6000- and 98000-Da molecular weight cut-off (MWCO). The ultrafiltration transmembrane pressures ranged from 0.5 to 4 bar and the feed flowrate from 100 to 200 1/h. The more hydrophobic component (the ellagic acid) is almost totally retained and that independently of the membrane MWCO, transmembrane pressure, and feed flowrate leading to the conclusion of formation of an adsorption layer responsible for membrane fouling.