An ultrasonic technique has been applied as a non-destructive, real-time, in situ measuring technique for the non-invasive study of fouling and cleaning during ultrafiltration (UF) with polysulfone (PSU) membranes. Paper mill effluent from a wastewater treatment plant was used as a feed solution. Firstly, an asymmetric, composite polysulfone membrane, its compaction and fouling were detected by the ultrasonic technique. The experimental results showed a good correspondence between the ultrasonic signal responses and the development of the fouling layer on the membrane surface. Secondly, the ultrasonic technique was successfully used for monitoring membrane cleaning and evaluating the cleaning effectiveness of various cleaning methods. Moreover, the technique provided a means of producing a differential signal (an echo signal of a fouling layer) by comparing reference and test waveforms. The differential signal indicates the state and progress of the fouling layer on a membrane surface and gives warning of advanced fouling during realistic operation. Results showed the nature and thickness of the fouling layer as a function of operating time. Traditional flux measurements and analysis of the membrane surface by microscopy corroborated the ultrasonic response results. The ultrasonic technique is a useful technique for the non-destructive investigation of fouling and cleaning in membrane applications. (C) 2003 Elsevier Science B.V. All rights reserved.