Monitoring membrane fouling is a key issue in operating a membrane water treatment plant. Most fouling identification methods are based on either permeate volume or pressure drop. In this study, two new parameters, "fouling weight" and "fouling rate" were proposed to quantify foulant on the membrane as well as its behaviour in membrane filtration. The parameters could be determined experimentally by the thermogravimetric method. Experiments were carried out to assess the accuracy and reliability of these parameters. It was found out that fouling weight and fouling rate were linearly correlated to the resistance of the fouling layer and the rate of fouling layer building up, respectively. Furthermore, the value of fouling rate was equal to the concentration of the fraction of solute that formed fouling layer on membrane. It was revealed that fouling weight and fouling rate could provide quantitative information of the interaction between the membrane and the impurities in the water. (C) 2002 Elsevier Science B.V. All rights reserved.