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This study explores the combined removal of four pesticides (atrazine, simazine, diuron, isoproturon) and hardness from groundwater with nanofiltration (NF). Four NF membranes were used: NF45 and NF70 (Dow/FilmTec), and UTC-20 and UTC-60 (Toray Ind. Inc.). Each pesticide was added to groundwater samples in a concentration of 1 μg/l, representative for contaminated sources. For the determination of pesticides in such low concentrations, an analytical method using solid-phase extraction and high performance liquid chromatography was elaborated. The retentions with NF were compared and explained by means of molecular size and dipole moment. Molecular size, expressed as an effective diameter representing the projection of the molecule on the membrane surface, was calculated by determining the energetically most favourable molecular configuration. The largest molecules had the highest retentions, when all other properties are comparable. A high dipole moment decreases retentions because of charge interactions with the membrane. The hardness retentions were high. For drinking water production, it might be necessary to add hardness after filtration. The groundwater flux was approximately 5% lower compared to the distilled water flux. This relatively small flux decline indicates pore blocking due to adsorption in the membrane pores. The flux decline was proved reversible; no membrane fouling occurred. [ABSTRACT FROM AUTHOR]