In a recent paper [Susanto, Ulbricht, J. Membr. Sci. 266 (2005) 132], we showed that dextran does foul polyethersulfone (PES) ultrafiltration (UF) membranes by contact of the soln. with the membrane surface without flux through the membrane. In this work, dextran fouling was visualized using at. force microscopy (AFM) and quantified by ATR-IR spectroscopy and by the mass balance in simultaneous diffusion-adsorption measurements (SDAM). Good correlations have been found between the water flux redn. due to dextran adsorption and the quant. data for bound dextran on the PES membranes. Further, a pronounced effect of dextran size on adsorptive membrane fouling was identified. Contact angle and zeta potential measurements with non-porous films, where solute entrapment in pores can be ruled out, gave addnl. clear evidence for dextran binding on the PES surface. Complementary data for adsorption and fouling of porous membranes and non-porous films by the protein myoglobin indicated that the larger fouling tendency for protein than for dextran is due to a higher surface coverage of PES by the adsorbed biomacromol. layer. Data for batch UF confirm the conclusions from the static contact expts. because significant fouling is obsd. for PES membranes (more severe for myoglobin than for dextrans), while no fouling is seen for a cellulose-based UF membrane with the same nominal cut-off. Finally, two mechanisms for the attractive PES-dextran interaction - multiple hydrogen bonding involving the SO2 groups of PES and \"surface dehydration\" of the relatively hydrophobic PES - are discussed.