Three different nanofiltration membranes NF45. NF270 and NTR7450 were modified in water or lactic acid solution with stirring for 20 min. The modification was characterized by octanoic acid filtration and FTIR analysis. The flux of the NF270 membrane was almost unchanged and octanoic acid fouled the membrane permanently after modification. However, modification of the NF45 and NTR7450 membranes increased the water fluxes considerably. The best fluxes were obtained after the filtration of octanoic acid with UV irradiated membranes. The flux was almost two-fold and eight-fold for NF45 and NTR7450 membranes, respectively. The water flux after octanoic acid filtration was higher for all the modified NTR7450 membranes than the water flux before filtration. It seems that octanoic acid either adsorbs on the NF45 and NTR7450 membranes or influences the orientation of the membrane structure, especially after UV irradiation. According to FTIR spectra every membrane contained a layer of polysulfone similar to Udel polysulfone. Moreover, octanoic and lactic acids were found in the membranes after filtration and modification. After UV irradiation all the membranes had changes in their chemical structure. The binding of octanoic acid to the membrane was different for each of the tested membranes.