For more than 2 years, the extension of the Mery-sur-Oise plant has been producing water from the river Oise, using nanofiltration technology for a production capacity of 140,000 m(3)/d, a world premiere on surface water. After presenting the reasons for the nanofiltration choice and the treatment processes, we'll analyze the performances of the plant at its different stages: pretreatment upstream, nanofiltration itself and water quality in the distribution system. Performances are very satisfactory, especially for the two main objectives: elimination of organic matters and of pesticides. The consumers are also satisfied with the reduction of the chlorine taste (chlorine rate has been diminished by factor 2) and of calcium concentration. During these 2 years of operation, several tunings have been conducted to optimize the process, such as active pretreatment parameters and membrane cleaning process. The whole plant is automated and the process control is performing very efficiently. An automatic piloting system has been developed and has been operative since the beginning of 2002. So far, operation additional costs, compared to a traditional plant with refining using ozone and carbon, are lower than foreseen. Nanofiltration in Mery-sur-Oise turns out to have been the right choice and is a complete success.