Amsterdam Water Supply (AWS) intends to increase the capacity of the Leiduin production plant. In the existing plant (capacity 70×106 m3/y) pretreated Rhine River water was infiltrated in the dune area west of Amsterdam for artificial recharge and after a residence time of approximately 100 days extracted and posttreated to achieve drinking water quality. In the extension (capacity 13×106 m3/y) the pretreated (coagulation, sedimentation, filtration) Rhine River water was not infiltrated in the dunes, but was treated directly without soil passage. First, an additional pretreatment (ozonation, biological activated carbon filtration and slow sand filtration) was carried out, and finally reverse osmosis (RO) was used for desalination, hardness removal, disinfection and removal of pesticides and others micro-pollutants. Former research on removal of pesticides has already showed a removal of >99.5% with the ozonation/BACF preceding RO. To prove high retention of RO membranes as a second barrier and to examine the influence of aging of the membranes, several dosing experiments were carried out by AWS and Kiwa. During the period before March 1997, Toray SU710L membranes were used. From then on Fluid Systems 4821ULP membranes have been used in the RO pilot plant. The RO feed flow is 9 m3/h and the recovery is 85%. To compare the removal of pesticides and herbicides with the two different applied RO membranes, six dosing experiments were carried out: two with the Toray membranes and four with the Fluid Systems membranes. A cocktail of pesticides was dosed with a feed concentration of approximately 5 μg/l. The results of the test showed an equal retention for bentazon, DNOC and pirimicarb for both types of membranes. The removal of metamitron and metribuzin was substantially higher with the Fluid Systems membranes. As a result, it was concluded that RO is a second barrier for pesticides in this treatment concept as biological activated carbon filtration in the first barrier. Retention of the Fluid Systems membranes is higher than the retention of the Toray membranes. After 3 years of operation with the Fluid Systems membranes, no pesticide retention decline was observed.