Since 1999 the Mery-sur-Oise plant has operated using two processes in parallel: one, a membrane process using nanofiltration, and the other, a conventional biological process. For operational flexibility, it is desired to be able to transfer some of the sand-filtered ozonated water from the biological process to the membranes. Before carrying out this transfer, it was important to assess the role of ozonation on membrane fouling, by comparing the effects of ozonated water (SFOW) with non-ozonated water (SFW). Tests were performed, in spring and fall 2001, in a pilot unit comprising two nanofiltration lines in parallel. The first test emphasized the essential role of the pre-filter placed upstream of the membranes. This latter - supplied with ozonated water – released particles when saturated, linked to changes in the feed water quality. Despite very frequent pre-filter replacements, fouling proved to be greater for the membranes supplied by ozonated water. Even though the silt density index and total cell counts levels of SFOW were lower than SFW, ozonated water could increase membrane fouling. The action of ozone on the nature of organic matter appeared to be the main cause. Ozone modified the composition of natural organic matter, on one hand by reducing the hydrophobic fraction in favour of the hydrophilic fraction, which is harder for the membrane NF 200B to reject, and on the other hand by acting on the lyses of algal cells and the release of extracellular organic matter. The membrane autopsies confirmed that the deposit is essentially composed of organic matter. Images taken with a scanning electron microscope showed a more significant deposit for membranes fouled with SFOW. For the membrane fouled in spring, the images also revealed the presence of algae; for some, cells were lysed upon ozonation. HPSEC-UVA-DOC enabled the types of molecules to be identified according to size. Humic substances (1,000 to 5,000 daltons) and other low molecular weight components (400 to 600 daltons) represented a minor part of the fouling material with the major components being proteins (21,000 to 24,000 daltons). Also, humic substances were relatively more abundant in the membrane fouled with ozonated water in spring.