This work investigates the extent to which the rejection of a target substance is influenced by the addition of other substances. The alteration effects on the permeation of acetic acid through a aromatic polyamide membrane when combined with one of 27 further organic acids was studied. The operating conditions were kept constant during all investigations. It was found that the retention of a target substance was dependant on the other compounds in particular aromatics and short-chained monocarboxylic and dicarboxylic acids. Acids which significant influenced acetic acid retention were additionally combined with other target substances, formic and propionic acid. The results showed that, in general, it is only possible to increase the direction of the target substances rejection, a higher rejection can only be increased, a lower rejection can only be decreased. These results were used to create and investigate quaternary solutions which showed that, in principle, it is possible to intensify the increasing effect of retention. Using the experiment data it should be possible to determine the separation capacity by concentration complex mixtures through reverse osmosis (RO).