The aim of this paper is to describe a system for fouling monitoring of thermosiphon reboilers. This system is based on a static model of the fouling resistance. The latter has been constructed in three steps. First, a mathematical model of the fouling resistance based oil the overall heat transfer coefficient and on the inside film beat transfer coefficient is developed. Then, the overall heat transfer coefficient is obtained from the mass and energy balances of the reboiler. In the last step, the inside film heat transfer coefficient is determined from correlations with some process variables. The resulting model is a function of measured quantities and of two unknown parameters. A procedure for identification of the unknown parameters from data selected so that hypotheses behind the model are fulfilled is presented. The effectiveness of the resulting monitoring system is illustrated by processing industrial data. (C) 2003 Elsevier Ltd. All rights reserved.