This paper presents a detailed investigation of fouling mechanisms for ultrafiltration membranes with polysaccharides obtained by studying membrane-solute (static adsorption) and membrane-solute-solute interactions (ultrafiltration (UF)). Two polyethersulfone (PES) membranes and one stabilized cellulose (cellulosic) membrane with a nominal cut-off of 10 kg/mol and dextrans with av. molar mass (M) of 4, 10 and 15 kg/mol were used. The membranes before and after static adsorption of dextran were characterized by captive bubble contact angle and tangential streaming potential measurements as well as ultrafiltration sieving curves for polyethylene glycols. Significant water flux redns. (4-15%), which also correlated with dextran molar mass, and changes of the other membrane characteristics occurred after static dextran adsorption for the PES membranes. An empirical model to describe the correlation between the relative water flux redn. and the concn. of solute had also been proposed. In contrast, no significant changes could be detected for the cellulosic membrane. Significant membrane-solute interactions had also been confirmed in the ultrafiltration expts. with dextrans where irreversible fouling had been obsd. for the PES but not for the cellulosic membranes. The results provide fundamental information for a better understanding of fouling by polysaccharides. In particular, it had been confirmed that hydrophilic and neutral dextrans can significantly foul PES membranes via adsorption to the surface of the membrane polymer. On this basis, methods for control of this fouling can be properly developed.