Fouling is the most critical problem associated with membrane separations in liquid media. But it is difficult to control the inevitable membrane fouling because of its invisibility, especially on the inside surface of hollow fiber membranes. This study describes the extension of ultrasonic time-domain reflectometry (UTDR) for the real-time measurement of particle deposition in a single hollow fiber membrane. A transducer with a frequency of 10 MHz and polyethersulfone hollow fiber membranes with 0.8 mm inside diameter (ID) and 1.2 mm outside diameter (OD) were used in this study. The fouling experiments were carried out with 1.8 g/L kaolin suspension at flow rates 16.7 and 10.0 cm/s. The results show that UTDR technique is able to distinguish and recognize the acoustic response signals generated from the interfaces water/upper outside surface of the hollow fiber, lumen upside surface/water, water/lumen underside surface and lower outside surface/water in the single hollow fiber membrane module in pure water phase. The systemic changes of acoustic responses from the inside surfaces of the hollow fiber in the time- and amplitude-domain with operation time during the fouling experiments were detected by UTDR. It is associated with the deposition and formation of the kaolin layer on the inside surfaces. Further, the acoustic measurement indicates that the deposited fouling layer is denser on the lumen underside surface of the hollow fiber than that on the lumen upside surface as a result of weight. Moreover, it is found that the fouling layer grows faster on the inside surface of the hollow fiber at a flow rate of 10.0 cm/s than that at 16.7 cm/s due to the lower shear stress. The fouling layer formed is thicker at a flow rate of 10.0 cm/s than that at 16.7 cm/s. The flux decline data and SEM analysis corroborate the ultrasonic measurement. Overall, this study confirms that UTDR measurement will provide not only a new protocol for the observation of hollow fiber membrane fouling and cleaning, but also a quantitative approach to the optimization of the membrane bioreactor system. (C) 2008 Elsevier B.V. All rights reserved.