Process water from a Swedish thermomechanical pulp mill was purified by consecutive treatment by microfiltration, ultrafiltration and nanofiltration. In the present study the nanofiltration step was studied. The flux and retention of four nanofiltration membranes were compared. The NF99HF membrane (Alfa Laval, Lund, Sweden), had significantly higher flux and higher retention of lignin and charged compounds (measured as conductivity) compared to the others. The flux was 220 l/m2 h and the retention of calcium ions was 99% at 20 bar transmembrane pressure and a temperature of 50°C. At the same conditions, the conductivity (measured at 20°C) was decreased from 1 500 µS/cm in the feed solution to 150 µS/cm in the nanofiltration permeate.