Organic trace substances including pesticides are becoming more important in the drinking water treatment. Nanofiltration for drinking water treatment is mainly used for softening and the removal of humic substances. Recently nanofiltration is sometimes used also for the removal of organic trace substances. The mechanism of the trace substance transport through Nanofiltration membranes is widely unknown. Therefor it is not possible to predict pesticiderejections. Thus one focus of this thesis is the investigation of the rejection of organic trace substances through nanofiltration membranes. By measuring diffusion constants of several electrical uncharged pesticides it could be shown that the size of the molecules is a contributing factor. Due to attached water molecules it is not possible to calculate this "effective" molecule size by molecule modelling programs. To investigat the steric effects the rejection dependent of pH was investigated because the membrane pore size distribution depends on pH. The oppositional behaviour of the rejection and the permeability underlines the steric influence on rejection as well as a mathematical model. With electric charged organic trace substances the electrostatic interactions between the membrane surface and the organic molecule dominates the transport mechanism.