A study on a nanofiltration pilot with synthetic waters was carried out in order to evaluate the performances of two nanofiltration membranes in the removal of some selected pesticides ( triazines and urea herbicides). The influence of the presence of humic acid, sulfate and chloride ions on the removal of these pesticides was shown. The results show very different behaviours between the two membranes. With a Desal DK membrane, only diuron was affected negatively by the matrix change. But with an NF200 membrane, the effects were more perceptible and led to a better comprehension of the mechanisms which govern the removal of pesticides by nanofiltration.