Nanofiltration (NF) membranes have found more frequent use in recent years for the desalination of seawater and other sources of brackish water because they can be used at lower pressures than more traditional reverse-osmosis (RO) technologies, and thus provide overall energy savings. However, membrane fouling still presents a common and significant challenge in practical applications. Currently, the performance of membrane-based liquid separation processes is most often monitored by external, volumetric flow-based techniques that provide delayed information on fouling layer development. The delay between initial growth and the observation of fully established fouling reduces the efficacy of cleaning and remediation measures. The focus of this study is the use of ultrasonic time-domain reflectometry (UTDR) as a non-destructive method for real-time, in-situ monitoring of early-stage inorganic scaling layer formation on NF membranes. This work utilizes miniature-scale ultrasonic transducers that are internally integrated into a flat-sheet cross-flow filtration module and in contact with the membrane. Comparisons are made with results obtained from externally mounted UTDR transducers, a more commonly used arrangement. Results show that while the internal sensors can be somewhat more sensitive, the significance of this improvement can be negated by scaling deposition that is hindered by the presence of the sensor. (C) 2011 Elsevier B.V. All rights reserved.