The ultrasonic control of membrane fouling caused by natural organic matter (NOM) and silica particles was investigated under various solution conditions. Twenty kilohertz ultrasound was applied to a cross-flow ultrafiltration system with gamma-alumina membranes. Experimental results indicated that solution chemistry affected the ability of ultrasound to control membrane fouling. More effective control of membrane fouling occurred at high pH, low ionic strength, and in the absence of divalent cations, consistent with the expectation of stronger electrostatic repulsion between foulant-foulant and foulant-membrane components. Fourier transform infrared spectra of the NOM foulant showed the presence of aliphatic carbon and carboxylate functional groups, both of which decreased in magnitude with increasing pH, in the absence of Ca2+, or the presence of ultrasound. A decrease in the NOM rejection rate during ultrasound was observed. This likely occurred due to decreased steric exclusion of NOM through the fouled membrane, as scanning electron microscope images indicated that ultrasound partially removed the foulant layer from the membrane surface allowing larger NOM molecules to pass through the membrane. No damage to the membrane by ultrasound was observed. (c) 2005 Elsevier B.V. All rights reserved.