Two quaternary NH4 salts in aq. solns. (concen. 50-1000 mg/L) were sepd. by nanofiltration through flat-sheet membranes and tubular modules to det. sepn. efficiency and membrane permeability. The pressure-driven membrane process was found as effective method for removal of the cationic surfactants. The highest retention coeffs. were 94 and 80% for compds. tested. The presence of surface active agents in the treated solns. affected hydraulic properties of the membranes.