The fouling of microfiltration membranes by natural brown-coloured surface water was studied in situ by ultrasonic time-domain reflectometry. An ultrasonic measurement system consisting of a 7.5 MHz ultrasonic transducer, a pulser-receiver, a digital oscilloscope and a computer was connected to a I aboratory-scale, flat-bed filtration module. The ultrasonic spectra were presented by line graphs and then transformed into wavelets. The latter presents the frequency spectrum of ultrasonic signals as the same function of time as in the line graphs. The ultrasonic frequency data indicated the first change at the membrane surface at 10s. This could be seen more clearly by subtracting the zero-time waveform from the subsequent waveforms. The entire fouling process and fouling layer growth could be more clearly and qualitatively monitored by applying wavelets to the ultrasonic reflection data, which normally gives less quantitative factual data. Vertical lines on the time axis or horizontal lines on the frequency axis can be drawn to intersect the fouling peak and this data can then be extracted to plot amplitude of the fouling peak versus operation time or versus frequency. Such new curves portraying the growth of the fouling peak, even at extremely short-fouling times, provide interesting data on the fouling process. These ultrasonic results were supported by flux measurements and scanning electron microscopy (SEM) examinations of the original and fouled membrane surfaces. (c) 2005 Elsevier B.V. All rights reserved.