Experiments to determine fouling layer composition have been conducted on three different beers. (i) a standard lager; (ii) a commercial lager; and (iii) a standard ale. Fresh Millipore membranes, nominal pore size 0.45 mum, were fouled by applying a transmembrane pressure of 0.7 barg. at 0 degreesC in a stirred cell. Each beer was tested both pre- and post-centrifugation. In each experiment, the flux decline data was obtained and then the fouling layer was removed using a sonicator. The fouling layers were freeze-dried and re-suspended in 0.1 M imidazole. They were then analysed for carbohydrate, polyphenols and protein. Unfiltered samples of each beer, both pre- and post-centrifugation were also analysed using the same procedure. The flux decline data show that the presence of yeast enhances flux. Furthermore, the final fluxes obtained for Beer 2, the commercial beer, were an order of magnitude greater than those obtained for the non-commercial beers. Analysis of the fouling layers reveals that for all beers, polyphenol is present in disproportionately greater quantities than are present in the unfiltered beer, while carbohydrate is present in disproportionately smaller amounts. The data for Beers 2 and 3 also suggest that protein is present in disproportionately greater quantities. It is further concluded that, for all beers, the fouling layer consists of both protein-polyphenol complexes and carbohydrate gels.