The demand for clean water is currently increasing because of the decreasing sources of drinking water the stringent environmental legislation and the rigorous quality demands. Nanofiltration may be a solution for the effective separation of anionic detergents from used water or wastewater. In this work the removal of an anionic surfactant cleaning agent by nanofiltration was investigated. The nanofiltration of aqueous solutions of the anionic surfactant was examined at detergent concentrations of 0.5, 1.0 and 5.0 g L-1 at 20, 30 and 40 degrees C and at 20, 30 and 40 bar. Statistical evaluation of the data revealed that the retention was principally affected by the temperature. The highest retention was observed at 20 degrees C. Increase of both pressure and temperature increased the flux, but the impact of pressure was higher than that of temperature. The highest flux was measured at 40 degrees C. There was no appreciable difference between 30 and 40 bar. The fouling index, calculated by fitting a power function to the measured data, was mainly affected by the temperature: it decreased with increasing temperature. The ideal parameters for the best retention and a reasonable flux were found to be 20 degrees C and 30 bar and for the minimal fouling were 30 degrees C and 30 bar.