Fouling of microfiltration membranes by beer components has been studied using a model feed solution developed from a knowledge of components and their concentrations found in beer. The model solution contained two long chain components, casein (protein) and starch (polysaccharide), as well as water, ethanol, glycerol, maltose, citric acid, calcium ions, catechin and ethyl acetate. Cross-flow filtration was performed with 0.2 mum flat sheet cellulose nitrate membranes, at a temperature of 20 degreesC, cross-flow velocities in the range 1.1-2.4 m s(-1) and transmembrane pressures in the range 1.9-4.5 bar. Proteins and polysaccharides are shown to cause rapid flux declines, although the polysaccharide dominated this behaviour when both components were present. The presence of the protein dominated the rejection levels of both maltose and catechin (polyphenol) due to polyphenol-protein interactions. Ca2+ affects permeate flux due to Ca2+-protein and Ca2+-polysaccharide interactions. There is also an apparent citric acid-protein interaction that affects citric acid rejection by the membrane.