A major problem in the drinking water production is the presence of pesticides in raw water, not only in surface waters, but also in ground waters. Concentrations of several mu g/l have been found. Traditionally, pesticides are removed using granular or powdered activated carbon, which is an expensive process. The cost is enhanced by competition of pesticides and natural organic matter (NOM) for adsorption sites on the activated carbon. Nanofiltration (NF) can be a valuable alternative for the removal of pesticides. Moreover, pesticides and hardness can be removed in one step. In this paper, the retention of four pesticides (atrazine, simazine, diuron, isoproturon) was determined experimentally for four NF membranes: NF-70 (Dow/FilmTec), NF-45 (Dow/FilmTec), UTC-20 (Toray Inc.) and NTR-7450 (Nitto-Denko). The experimental retentions indicate that NF-70 is a suitable membrane for removal of pesticides. The experimental retentions were related to structural properties of the molecules. The size of the molecule is the most important parameter, and may be represented by molecular weight. or more accurately by a molecular diameter calculated from the molecular structure. Furthermore, a comparison with retentions of a series of saccharides confirms the earlier found effect of the dipole moment: a higher dipole moment results in a lower retention. No effect of the pesticide concentration was found. The matrix of the ground waters caused an increase of the pesticide retention, together with a decrease of the water flux through the membrane.