This paper investigates the influence of molecular size and hydrophobicity of dissolved organic compounds in aqueous solution on retention. In a first part, the influence of hydrophobicity was investigated for organic molecules with a similar molecular weight below the molecular weight cut-off (MWCO) of the membrane and a different hydrophobicity, expressed by the logarithm of the octanol-water partition coefficient (log P). A good correlation was found between the hydrophobicity and the retention: molecules with a high log P (hydrophobic) generally had a low retention, while molecules with a negative log P (hydrophilic) showed a high retention. This could be explained by hydration of the molecules. In a second part, the correlation between the retention of molecules with a molecular weight above the MWCO of the membrane and their hydrophobicity was studied. It was found that a high hydrophobicity lowers the retention, but the influence of hydrophobicity decreases if the molecular size (compared to the MWCO of the membrane) increases. Finally, the influence of adsorption on retention was investigated. An increase in permeate concentration and thus a decrease of the corresponding retention, as a function of time, was observed for hydrophobic compounds, until saturation of the membrane was reached (steady-state retention). (c) 2004 Elsevier B.V. All rights reserved.