Nanofiltration experiments were carried out in order to investigate the effects of the mixture ratio and the change in concentration polarization on the operating conditions, in particular, the cross-flow velocity for the mixed solutions of sodium chloride (NaCl) and monosodium glutamate (MSG). The rejection's of Cl- strongly depended on the mixture ratio but slightly on the concentration. The rejections of MSG were only slightly influenced by the mixture ratio and concentration. The apparent rejection of Cl- decreased with decreasing cross-flow velocity for the mixed solution in which the rejection of Cl-was positive (the concentration of Cl- was higher than that of MSG), and increased with decreasing cross-flow velocity for the mixed solution in which the rejection of Cl- was negative (the concentration of Cl- was lower than that of MSG). The effect of the cross-flow velocity on the apparent rejection of Cl- was reversed as the MSG concentration increased. These results suggest that the rejection of Cl- in the mixed solution could be governed by the accumulated solute of higher concentration. The change in the operating conditions appropriately corresponding to the mixture ratio with time would be required in order to effectively operate the concentration and/or diafiltration process.