Ultrasound effects at frequencies of 28, 45 and 100 kHz and at a power intensity in the range of 2.5-3.3 W/cm(2) on a permeate flux of dextran solutions through polyacrylonitrile ultrafiltration membranes were examined. When ultrasound was irradiated onto membranes, no effect was observed for water permeation. In the case of dextran solutions, ultrasound treatment enhanced permeation flux through membranes, and the effect varied at different frequencies. It was also found that the extent of the enhancement of permeate flux increased with increasing ultrasound intensity, and the irradiation direction of ultrasound was also an important factor. Evidence was presented that ultrasound irradiation enhanced permeation by increasing mass transfer across the concentrated dextran layer close to the membrane surface. The reduction of the solute concentration at the membrane surface lowered the osmotic pressure, which was also beneficial to permeation through the membrane.