Ultrafiltration experiments on solutions of glucose, pectin, and their mixtures have been performed in a 500 1/h pilot scale crossflow membrane unit. A 25,000 MWCO polyethersulfone membrane with a total flow area of 0.9 m(2) was used in the process. Unlike the permeate flux profiles for glucose solutions, which showed linear relationship with transmembrane pressure (TMP), the flux for solutions of pectin showed a rapid increase with increasing TMP before leveling-off. Similar behavior was observed by adding different amounts of glucose to these pectin solutions, but with much lower permeate flows. The formation of gel layers on the membrane surface is mainly responsible for the lower permeate fluxes in the latter two cases. In addition, it was found that particle interactions due to electrostatic forces were also important for the development of stable gel layers. Due to differences in the experimental technique used in this work, no maximum permeate flux for the pectin solutions was observed at intermediate TMP as opposed to many results reported in the literature.