Fouling is the major issue when using membranes for water treatment. Several parameters have been proposed for measuring a fouling potential and using it as a predictive tool for assessing the adequacy of pre-treatment. Up to now, the Silt Density Index and the MFI0.45 (Modified Fouling Index) are used, but they do not reflect the real potential of fouling. In fact, particles smaller than 0.45 mu m responsible for fouling are not taken into consideration. An improvement of the determination of the fouling index is proposed by using NF membranes and strictly defined working conditions (constant transmembrane pressure).