The effect of temperature on the retention in nanofiltration of model substances (glucose) and substances in industrial streams has been studied in the temperature interval 25degreesC to 65degreesC. An increase in temperature decreased the retention until a critical temperature of the membrane was exceeded. After that temperature the flux even decreased and the retention increased. The retention of uncharged substances normally decreased due to alkaline cleaning and the flux of most membranes increased. In the filtration of mechanical pulp mill effluents an increase of temperature decreased the retention of organic substances but did not affect the retention of conductivity. Only few nanofiltration membranes withstood a temperature of 65degreesC without a significant change in the filtration efficiency. A membrane pre-treatment, such as pressurization with pure water affected dramatically the flux and the flux reduction and somewhat the retention. Experiments made with incompletely wetted membranes underestimate fouling and the retention of the membrane. Most nanofiltration membranes seemed to be almost completely wetted at a pressure of 25 bar.