In this paper the softening of groundwater with drinking-water quality using nanofiltration is studied. Experiments were carried out with three commercial nanofiltration membranes. The UTC 20 membrane was chosen for further experiments. The influence of water temperature was studied on both permeate flux and ion retention. Also, the effect of a higher recovery was studied: experimental and predicted values were compared for recoveries of 0 to 70%. Finally, experiments were carried out to study membrane fouling. The use of hydrochloric and sulphuric acid was compared and the effect of an ultrafiltration pretreatment on the water flux was examined.