The aim of this paper is to describe a system for fouling monitoring in thermosiphon reboilers. This system is based on a static model of the fouling resistance of which the parameters are identified from industrial data. The determination of adequate data requires the detection of transients due to set point changes, physical disturbances or change in fouling, as well as the selection of sufficiently informative measurements. These two tasks are performed by combining statistical change detection tests, and a measure of information content in a suitable decision system. The effectiveness of the method is illustrated by processing industrial data.