In this paper the separation of aqueous organic multi-component solutions through a polyamide reverse osmosis membrane was systematically investigated. To analyze coupling effects and competing reactions between substances at or inside the membrane and to determine the influence of the solution components on the separation behaviour, experiments with model solutions were carried out. For the model solutions one of 34 different organic substances was added to an aqueous l-propanol solution. The substances were chosen due to their environmental relevance and regarding their physico-chemical properties. The observed changes in separation and mass transfer of l-propanol were used to develop a general mass transport model for reverse osmosis. This model was based on the Kimura-Sourirajan Analysis (KSA) developed by Sourirajan in 1963. It was modified to include the interactions among different substances in multi-component solutions by means of their physico-chemical properties. The model describes the mass transport through a reverse osmosis membrane and enables the prediction of the retention behaviour of organic multi-component solutions.