The fouling of plate heat exchangers (PHE) is a serious problem in the food industry. Fouling reduces both PHE efficiency and food quality and can also give rise to microbiological problems. In order to monitor fouling inside PHE in real time, a new acoustic technique, using multiple optimized non-intrusive sensors, was developed for experimental on-line trials. Low-frequency acoustic waves were propagated through the plates, and these waves were analyzed to measure their ability to detect fouling inside the PHE. By comparing the evolution of the acoustic wave parameters (e.g. power and delay), an indicator of the fouling rate is obtained for each zone inside the exchanger. The results of this analysis show that low-frequency acoustic waves are sensitive to PHE fouling. (C) 2007 Elsevier Ltd. All rights reserved.