The filtration of organic compounds in aqueous solutions through composite reverse osmosis membranes has been investigated. The examined solutions also contained inorganic salts; organic compounds were represented by benzoic acid derivatives. Our present work was aimed at clarifying the behaviour of the aforementioned compounds among conditions of RO filtration and we were seeking an answer to the question of how high the influence of counter-ions can be on the relative retentions. The partition coefficients of organic compounds ranged from 0.19 to 0.32 while those of simple inorganic salts were found between 0.43 and 0.51. The variation of alkali counter-ion exerted a moderate influence on the partition coefficients of an anionic partner, while the effect of highly substituted ammonium ions seemed to be larger. The ratio of partition coefficients of two solutes varied from 0.19 to 0.68, and some difference of retention was caused by the ring substituents of benzoic acid. The flux was found to be a linear function of the partition coefficients of solutes. An organic component of a multicomponent solution can be concentrated this way. We intend to explain our experimental results with respect to the molecular features of the corresponding salts and their ions.