The influence of chemical functionality in carboxylic acids and osmotic pressure of multi-component solutions on the retention in the reverse osmosis process are discussed. Therefore formic-, acetic-, propionic-, glycolic-, acrylic-and methoxyacetic acid (target substances) were combined with one or two other carboxylic acids (active substances), chosen out of a pool of 16. All investigations were carried out with an aromatic polyamide membrane and the operating conditions were kept constant. Although the combination of all effects is extremely complex, the experiments showed that the influence on the retention of a substance equate as an outcome of molecular mass, acidity, functionality and spatial requirement. The influence of functionality in the active substances could be further divided into an additional carboxylic group, double bond and aromaticity. The calculation of the osmotic pressure demonstrated that there was no observable difference between the real and ideal one when the solution contained the same number of components. Therefore it can be concluded that in this study the osmotic pressure has no influence on the retention of the target substance.